25 files changed, 391 insertions, 161 deletions

diff --git a/subprojects/frontend/src/editor/EditorTheme.ts b/subprojects/frontend/src/editor/EditorTheme.ts
index 1cad4a36..f499b0d9 100644
--- a/subprojects/frontend/src/editor/EditorTheme.ts
+++ b/subprojects/frontend/src/editor/EditorTheme.ts
@@ -126,7 +126,7 @@ export default styled('div', {
         color: theme.palette.text.secondary,
       },
     },
-    '.tok-problem-individual': {
+    '.tok-problem-atom': {
       '&, & .tok-variableName': {
         color: theme.palette.text.primary,
       },
diff --git a/subprojects/frontend/src/language/problem.grammar b/subprojects/frontend/src/language/problem.grammar
index ce3baa02..b08a9c36 100644
--- a/subprojects/frontend/src/language/problem.grammar
+++ b/subprojects/frontend/src/language/problem.grammar
@@ -28,7 +28,7 @@ statement {
     (":" Expr)? "."
   } |
   ProblemDeclaration {
-    kw<"problem"> QualifiedName "."
+    (ckw<"module"> | kw<"problem">) QualifiedName "."
   } |
   ClassDefinition {
     kw<"abstract">? kw<"class"> RelationName
@@ -37,7 +37,7 @@ statement {
   } |
   EnumDefinition {
     kw<"enum"> RelationName
-    (EnumBody { "{" sep<",", IndividualNodeName> "}" } | ".")
+    (EnumBody { "{" sep<",", AtomNodeName> "}" } | ".")
   } |
   PredicateDefinition {
     (
@@ -56,8 +56,11 @@ statement {
   //  RuleName ParameterList<Parameter>?
   //  RuleBody { ":" sep<OrOp, Conjunction> "==>" sep<OrOp, Consequent> "." }
   //} |
-  IndividualDeclaration {
-    kw<"indiv"> sep<",", IndividualNodeName> "."
+  AtomDeclaration {
+    ckw<"atom"> sep<",", AtomNodeName> "."
+  } |
+  NodeDeclaration {
+    (ckw<"node"> | ckw<"multi">) sep<",", NodeName> "."
   } |
   ScopeDeclaration {
     kw<"scope"> sep<",", ScopeElement> "."
@@ -161,7 +164,7 @@ RelationName { QualifiedName ~name }
 
 //RuleName { QualifiedName }
 
-IndividualNodeName { QualifiedName }
+AtomNodeName { QualifiedName }
 
 VariableName[@dynamicPrecedence=10] { QualifiedName ~name }
 
diff --git a/subprojects/frontend/src/language/problemLanguageSupport.ts b/subprojects/frontend/src/language/problemLanguageSupport.ts
index 2121e05f..3847fdd8 100644
--- a/subprojects/frontend/src/language/problemLanguageSupport.ts
+++ b/subprojects/frontend/src/language/problemLanguageSupport.ts
@@ -27,7 +27,8 @@ const parserWithMetadata = parser.configure({
     styleTags({
       LineComment: t.lineComment,
       BlockComment: t.blockComment,
-      'problem class enum pred fn indiv scope': t.definitionKeyword,
+      'module problem class enum pred fn scope': t.definitionKeyword,
+      'node atom multi': t.definitionKeyword,
       'abstract extends refers contains container opposite': t.modifier,
       'default error contained containment': t.modifier,
       'true false unknown error': t.keyword,
@@ -44,7 +45,7 @@ const parserWithMetadata = parser.configure({
       String: t.string,
       'RelationName/QualifiedName': t.typeName,
       // 'RuleName/QualifiedName': t.typeName,
-      'IndividualNodeName/QualifiedName': t.atom,
+      'AtomNodeName/QualifiedName': t.atom,
       'VariableName/QualifiedName': t.variableName,
       '{ }': t.brace,
       '( )': t.paren,
@@ -54,7 +55,8 @@ const parserWithMetadata = parser.configure({
     }),
     indentNodeProp.add({
       ProblemDeclaration: indentDeclaration,
-      UniqueDeclaration: indentDeclaration,
+      AtomDeclaration: indentDeclaration,
+      NodeDeclaration: indentDeclaration,
       ScopeDeclaration: indentDeclaration,
       PredicateBody: indentPredicateOrRule,
       FunctionBody: indentPredicateOrRule,
@@ -70,7 +72,8 @@ const parserWithMetadata = parser.configure({
       // RuleBody: foldInside,
       Conjunction: foldConjunction,
       // Consequent: foldWholeNode,
-      UniqueDeclaration: foldDeclaration,
+      AtomDeclaration: foldDeclaration,
+      NodeDeclaration: foldDeclaration,
       ScopeDeclaration: foldDeclaration,
       BlockComment: foldBlockComment,
     }),
diff --git a/subprojects/language-ide/src/main/java/tools/refinery/language/ide/syntaxcoloring/ProblemSemanticHighlightingCalculator.java b/subprojects/language-ide/src/main/java/tools/refinery/language/ide/syntaxcoloring/ProblemSemanticHighlightingCalculator.java
index f64d4066..94a61c4f 100644
--- a/subprojects/language-ide/src/main/java/tools/refinery/language/ide/syntaxcoloring/ProblemSemanticHighlightingCalculator.java
+++ b/subprojects/language-ide/src/main/java/tools/refinery/language/ide/syntaxcoloring/ProblemSemanticHighlightingCalculator.java
@@ -29,7 +29,7 @@ public class ProblemSemanticHighlightingCalculator extends DefaultSemanticHighli
         private static final String CONTAINMENT_CLASS = "containment";
         private static final String ERROR_CLASS = "error";
         private static final String NODE_CLASS = "node";
-        private static final String INDIVIDUAL_NODE_CLASS = "individual";
+        private static final String ATOM_NODE_CLASS = "atom";
         private static final String NEW_NODE_CLASS = "new";
 
         @Inject
@@ -137,10 +137,10 @@ public class ProblemSemanticHighlightingCalculator extends DefaultSemanticHighli
                 if (reference == ProblemPackage.Literals.VARIABLE_OR_NODE_EXPR__VARIABLE_OR_NODE) {
                         classesBuilder.add(NODE_CLASS);
                 }
-                if (ProblemUtil.isIndividualNode(node)) {
-                        classesBuilder.add(INDIVIDUAL_NODE_CLASS);
+                if (ProblemUtil.isAtomNode(node)) {
+                        classesBuilder.add(ATOM_NODE_CLASS);
                 }
-                if (ProblemUtil.isNewNode(node)) {
+                if (ProblemUtil.isMultiNode(node)) {
                         classesBuilder.add(NEW_NODE_CLASS);
                 }
         }
diff --git a/subprojects/language-model/problem.aird b/subprojects/language-model/problem.aird
index a307ae83..a3420a04 100644
--- a/subprojects/language-model/problem.aird
+++ b/subprojects/language-model/problem.aird
@@ -1,25 +1,25 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <xmi:XMI xmi:version="2.0" xmlns:xmi="http://www.omg.org/XMI" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:description="http://www.eclipse.org/sirius/description/1.1.0" xmlns:description_1="http://www.eclipse.org/sirius/diagram/description/1.1.0" xmlns:diagram="http://www.eclipse.org/sirius/diagram/1.1.0" xmlns:ecore="http://www.eclipse.org/emf/2002/Ecore" xmlns:notation="http://www.eclipse.org/gmf/runtime/1.0.3/notation" xmlns:style="http://www.eclipse.org/sirius/diagram/description/style/1.1.0" xmlns:viewpoint="http://www.eclipse.org/sirius/1.1.0" xsi:schemaLocation="http://www.eclipse.org/sirius/description/1.1.0 http://www.eclipse.org/sirius/1.1.0#//description http://www.eclipse.org/sirius/diagram/description/1.1.0 http://www.eclipse.org/sirius/diagram/1.1.0#//description http://www.eclipse.org/sirius/diagram/description/style/1.1.0 http://www.eclipse.org/sirius/diagram/1.1.0#//description/style">
-  <viewpoint:DAnalysis uid="_CqOewKA4EeuqkpDnuik1sg" selectedViews="_CsAAYKA4EeuqkpDnuik1sg" version="15.0.0.202201261500">
+  <viewpoint:DAnalysis uid="_CqOewKA4EeuqkpDnuik1sg" selectedViews="_CsAAYKA4EeuqkpDnuik1sg" version="15.2.0.202303281325">
     <semanticResources>src/main/resources/model/problem.ecore</semanticResources>
     <semanticResources>src/main/resources/model/problem.genmodel</semanticResources>
     <semanticResources>build/resources/main/model/problem.ecore</semanticResources>
     <semanticResources>build/resources/main/model/problem.genmodel</semanticResources>
     <ownedViews xmi:type="viewpoint:DView" uid="_CsAAYKA4EeuqkpDnuik1sg">
       <viewpoint xmi:type="description:Viewpoint" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']"/>
-      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_CsYa4KA4EeuqkpDnuik1sg" name="declarations" repPath="#_CsUwgKA4EeuqkpDnuik1sg" changeId="7fa3b02a-a620-4e42-87d5-b2cc35ee8070">
+      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_CsYa4KA4EeuqkpDnuik1sg" name="declarations" repPath="#_CsUwgKA4EeuqkpDnuik1sg" changeId="1706658830410">
         <description xmi:type="description_1:DiagramDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']"/>
         <target xmi:type="ecore:EPackage" href="src/main/resources/model/problem.ecore#/"/>
       </ownedRepresentationDescriptors>
-      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_z1bOYF3lEe2LuOZzJ_LhLg" name="expressions" repPath="#_z1YyIF3lEe2LuOZzJ_LhLg" changeId="847d1bab-b9a3-4b3c-9007-d1ef6cbc7d2b">
+      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_z1bOYF3lEe2LuOZzJ_LhLg" name="expressions" repPath="#_z1YyIF3lEe2LuOZzJ_LhLg" changeId="1706657349781">
         <description xmi:type="description_1:DiagramDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']"/>
         <target xmi:type="ecore:EPackage" href="src/main/resources/model/problem.ecore#/"/>
       </ownedRepresentationDescriptors>
-      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_xj9yMF4eEe2rXNsIDUvqhw" name="assertions" repPath="#_xj6H0F4eEe2rXNsIDUvqhw" changeId="af7c7d5e-5f27-4ef1-b9ba-f5b0e7934a73">
+      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_xj9yMF4eEe2rXNsIDUvqhw" name="assertions" repPath="#_xj6H0F4eEe2rXNsIDUvqhw" changeId="1706657349781">
         <description xmi:type="description_1:DiagramDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']"/>
         <target xmi:type="ecore:EPackage" href="src/main/resources/model/problem.ecore#/"/>
       </ownedRepresentationDescriptors>
-      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_sCFokF4fEe2rXNsIDUvqhw" name="actions" repPath="#_sCFBgF4fEe2rXNsIDUvqhw" changeId="46042178-5db9-429c-bfeb-2f746d757073">
+      <ownedRepresentationDescriptors xmi:type="viewpoint:DRepresentationDescriptor" uid="_sCFokF4fEe2rXNsIDUvqhw" name="actions" repPath="#_sCFBgF4fEe2rXNsIDUvqhw" changeId="1706657349781">
         <description xmi:type="description_1:DiagramDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']"/>
         <target xmi:type="ecore:EPackage" href="src/main/resources/model/problem.ecore#/"/>
       </ownedRepresentationDescriptors>
@@ -31,6 +31,10 @@
         <children xmi:type="notation:Node" xmi:id="_D1D6MKA4EeuqkpDnuik1sg" type="2003" element="_D05iIKA4EeuqkpDnuik1sg">
           <children xmi:type="notation:Node" xmi:id="_D1EhQKA4EeuqkpDnuik1sg" type="5007"/>
           <children xmi:type="notation:Node" xmi:id="_D1FIUKA4EeuqkpDnuik1sg" type="7004">
+            <children xmi:type="notation:Node" xmi:id="_ep380L_JEe6Mo_-4--GvQg" type="3010" element="_eplB4L_JEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_ep380b_JEe6Mo_-4--GvQg" fontColor="2697711" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_ep380r_JEe6Mo_-4--GvQg"/>
+            </children>
             <styles xmi:type="notation:SortingStyle" xmi:id="_D1FIUaA4EeuqkpDnuik1sg"/>
             <styles xmi:type="notation:FilteringStyle" xmi:id="_D1FIUqA4EeuqkpDnuik1sg"/>
           </children>
@@ -216,11 +220,15 @@
         <children xmi:type="notation:Node" xmi:id="_pdJrwAGyEey7cfH5K6RyCw" type="2003" element="_pcXBkAGyEey7cfH5K6RyCw">
           <children xmi:type="notation:Node" xmi:id="_pdOkQAGyEey7cfH5K6RyCw" type="5007"/>
           <children xmi:type="notation:Node" xmi:id="_pdOkQQGyEey7cfH5K6RyCw" type="7004">
+            <children xmi:type="notation:Node" xmi:id="_u4zNEL_KEe6Mo_-4--GvQg" type="3010" element="_u4PMYL_KEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_u4zNEb_KEe6Mo_-4--GvQg" fontColor="2697711" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_u4zNEr_KEe6Mo_-4--GvQg"/>
+            </children>
             <styles xmi:type="notation:SortingStyle" xmi:id="_pdOkQgGyEey7cfH5K6RyCw"/>
             <styles xmi:type="notation:FilteringStyle" xmi:id="_pdOkQwGyEey7cfH5K6RyCw"/>
           </children>
           <styles xmi:type="notation:ShapeStyle" xmi:id="_pdJrwQGyEey7cfH5K6RyCw" fontName="Noto Sans" fontHeight="8"/>
-          <layoutConstraint xmi:type="notation:Bounds" xmi:id="_pdJrwgGyEey7cfH5K6RyCw" x="104" y="524" width="147" height="100"/>
+          <layoutConstraint xmi:type="notation:Bounds" xmi:id="_pdJrwgGyEey7cfH5K6RyCw" x="131" y="524" width="120" height="100"/>
         </children>
         <children xmi:type="notation:Node" xmi:id="_A9YrQCrZEeyyC-O0_LlY9w" type="2003" element="_A8hIkCrZEeyyC-O0_LlY9w">
           <children xmi:type="notation:Node" xmi:id="_A9c8sCrZEeyyC-O0_LlY9w" type="5007"/>
@@ -432,6 +440,44 @@
           <styles xmi:type="notation:ShapeStyle" xmi:id="_pOTU4Wg8Ee25oofngfVl_A" fontName="Noto Sans" fontHeight="8"/>
           <layoutConstraint xmi:type="notation:Bounds" xmi:id="_pOTU4mg8Ee25oofngfVl_A" x="776" y="1280" width="120" height="100"/>
         </children>
+        <children xmi:type="notation:Node" xmi:id="_ZcIsoL_JEe6Mo_-4--GvQg" type="2003" element="_ZbwSIL_JEe6Mo_-4--GvQg">
+          <children xmi:type="notation:Node" xmi:id="_ZcJTsL_JEe6Mo_-4--GvQg" type="5007"/>
+          <children xmi:type="notation:Node" xmi:id="_ZcJTsb_JEe6Mo_-4--GvQg" type="7004">
+            <children xmi:type="notation:Node" xmi:id="_cstskL_JEe6Mo_-4--GvQg" type="3010" element="_csN9UL_JEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_cstskb_JEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_cstskr_JEe6Mo_-4--GvQg"/>
+            </children>
+            <children xmi:type="notation:Node" xmi:id="_dVoGsL_JEe6Mo_-4--GvQg" type="3010" element="_dVQTQL_JEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_dVoGsb_JEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_dVoGsr_JEe6Mo_-4--GvQg"/>
+            </children>
+            <styles xmi:type="notation:SortingStyle" xmi:id="_ZcJTsr_JEe6Mo_-4--GvQg"/>
+            <styles xmi:type="notation:FilteringStyle" xmi:id="_ZcJTs7_JEe6Mo_-4--GvQg"/>
+          </children>
+          <styles xmi:type="notation:ShapeStyle" xmi:id="_ZcIsob_JEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+          <layoutConstraint xmi:type="notation:Bounds" xmi:id="_ZcIsor_JEe6Mo_-4--GvQg" x="131" y="20" width="120" height="100"/>
+        </children>
+        <children xmi:type="notation:Node" xmi:id="_aq0Q0L_KEe6Mo_-4--GvQg" type="2003" element="_aqfgsL_KEe6Mo_-4--GvQg">
+          <children xmi:type="notation:Node" xmi:id="_aq0Q07_KEe6Mo_-4--GvQg" type="5007"/>
+          <children xmi:type="notation:Node" xmi:id="_aq0Q1L_KEe6Mo_-4--GvQg" type="7004">
+            <children xmi:type="notation:Node" xmi:id="_cWM0kL_KEe6Mo_-4--GvQg" type="3010" element="_cV-yIL_KEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_cWM0kb_KEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_cWM0kr_KEe6Mo_-4--GvQg"/>
+            </children>
+            <children xmi:type="notation:Node" xmi:id="_cru_sL_KEe6Mo_-4--GvQg" type="3010" element="_crhkUL_KEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_cru_sb_KEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_cru_sr_KEe6Mo_-4--GvQg"/>
+            </children>
+            <children xmi:type="notation:Node" xmi:id="_c7vxoL_KEe6Mo_-4--GvQg" type="3010" element="_c7jkYL_KEe6Mo_-4--GvQg">
+              <styles xmi:type="notation:FontStyle" xmi:id="_c7vxob_KEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+              <layoutConstraint xmi:type="notation:Location" xmi:id="_c7vxor_KEe6Mo_-4--GvQg"/>
+            </children>
+            <styles xmi:type="notation:SortingStyle" xmi:id="_aq0Q1b_KEe6Mo_-4--GvQg"/>
+            <styles xmi:type="notation:FilteringStyle" xmi:id="_aq0Q1r_KEe6Mo_-4--GvQg"/>
+          </children>
+          <styles xmi:type="notation:ShapeStyle" xmi:id="_aq0Q0b_KEe6Mo_-4--GvQg" fontName="Noto Sans" fontHeight="8"/>
+          <layoutConstraint xmi:type="notation:Bounds" xmi:id="_aq0Q0r_KEe6Mo_-4--GvQg" x="131" y="716" width="120" height="100"/>
+        </children>
         <styles xmi:type="notation:DiagramStyle" xmi:id="_CsZB8qA4EeuqkpDnuik1sg"/>
         <edges xmi:type="notation:Edge" xmi:id="_4eaYwKA8EeuqkpDnuik1sg" type="4001" element="_4eU5TqA8EeuqkpDnuik1sg" source="_D1D6MKA4EeuqkpDnuik1sg" target="_xsq_MKA8EeuqkpDnuik1sg">
           <children xmi:type="notation:Node" xmi:id="_4ea_06A8EeuqkpDnuik1sg" type="6001">
@@ -675,18 +721,18 @@
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         <edges xmi:type="notation:Edge" xmi:id="_zKMY0AGyEey7cfH5K6RyCw" type="4001" element="_zJpmRgGyEey7cfH5K6RyCw" source="_pdJrwAGyEey7cfH5K6RyCw" target="_xsq_MKA8EeuqkpDnuik1sg">
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-            <layoutConstraint xmi:type="notation:Bounds" xmi:id="_zKM_4QGyEey7cfH5K6RyCw" x="-400" y="-16"/>
+            <layoutConstraint xmi:type="notation:Bounds" xmi:id="_zKM_4QGyEey7cfH5K6RyCw" x="-397" y="-12"/>
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-            <layoutConstraint xmi:type="notation:Bounds" xmi:id="_zKM_4wGyEey7cfH5K6RyCw" x="-23"/>
+            <layoutConstraint xmi:type="notation:Bounds" xmi:id="_zKM_4wGyEey7cfH5K6RyCw" x="-43"/>
           </children>
           <children xmi:type="notation:Node" xmi:id="_zKM_5AGyEey7cfH5K6RyCw" type="6003">
-            <layoutConstraint xmi:type="notation:Bounds" xmi:id="_zKM_5QGyEey7cfH5K6RyCw" x="-84" y="10"/>
+            <layoutConstraint xmi:type="notation:Bounds" xmi:id="_zKM_5QGyEey7cfH5K6RyCw" x="-88" y="10"/>
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           <styles xmi:type="notation:ConnectorStyle" xmi:id="_zKMY0QGyEey7cfH5K6RyCw" routing="Rectilinear"/>
           <styles xmi:type="notation:FontStyle" xmi:id="_zKMY0gGyEey7cfH5K6RyCw" fontColor="7490599" fontName="Noto Sans" fontHeight="8"/>
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+          <bendpoints xmi:type="notation:RelativeBendpoints" xmi:id="_zKMY0wGyEey7cfH5K6RyCw" points="[-21, -3, -828, 120]$[-21, -51, -828, 72]$[843, -51, 36, 72]$[843, -97, 36, 26]"/>
+          <sourceAnchor xmi:type="notation:IdentityAnchor" xmi:id="_zKNm8AGyEey7cfH5K6RyCw" id="(0.4576271186440678,0.030612244897959183)"/>
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@@ -750,7 +796,7 @@
           <styles xmi:type="notation:ConnectorStyle" xmi:id="_s7wgoQGyEey7cfH5K6RyCw" routing="Tree"/>
           <styles xmi:type="notation:FontStyle" xmi:id="_s7wgogGyEey7cfH5K6RyCw" fontName="Noto Sans" fontHeight="8"/>
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+          <sourceAnchor xmi:type="notation:IdentityAnchor" xmi:id="_s71ZIAGyEey7cfH5K6RyCw" id="(0.652542372881356,0.0)"/>
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@@ -1332,10 +1378,18 @@
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       <arrangeConstraints>KEEP_SIZE</arrangeConstraints>
       <arrangeConstraints>KEEP_RATIO</arrangeConstraints>
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+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_eplB4L_JEe6Mo_-4--GvQg" name="kind : ModuleKind = PROBLEM" tooltipText="">
+        <target xmi:type="ecore:EAttribute" href="src/main/resources/model/problem.ecore#//Problem/kind"/>
+        <semanticElements xmi:type="ecore:EAttribute" href="src/main/resources/model/problem.ecore#//Problem/kind"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_fPjcw7_JEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EClass']/@subNodeMappings[name='EC%20EAttribute']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EClass']/@subNodeMappings[name='EC%20EAttribute']"/>
+      </ownedElements>
     </ownedDiagramElements>
     <ownedDiagramElements xmi:type="diagram:DNodeList" uid="_JTstIKA4EeuqkpDnuik1sg" name="ClassDeclaration" tooltipText="" outgoingEdges="_jk6-PKA4EeuqkpDnuik1sg _plK3JqA9EeuqkpDnuik1sg _-XLKltawEeuymriYTNxK2g _m2GbcmTvEe2qdtyPWAtoxA _YWqZhmTzEe2qdtyPWAtoxA" width="12" height="10">
       <target xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//ClassDeclaration"/>
@@ -1728,12 +1782,12 @@
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       <target xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//Problem"/>
       <semanticElements xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//Problem"/>
-      <ownedStyle xmi:type="diagram:EdgeStyle" uid="_m-61UNXtEeuF_d0WEhR3Xw" targetArrow="InputClosedArrow" routingStyle="tree">
+      <ownedStyle xmi:type="diagram:EdgeStyle" uid="_fPrYkL_JEe6Mo_-4--GvQg" targetArrow="InputClosedArrow" routingStyle="tree">
         <description xmi:type="style:EdgeStyleDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@edgeMappings[name='EC%20ESupertypes']/@style"/>
-        <beginLabelStyle xmi:type="diagram:BeginLabelStyle" uid="_m-61UdXtEeuF_d0WEhR3Xw" showIcon="false">
+        <beginLabelStyle xmi:type="diagram:BeginLabelStyle" uid="_fPrYkb_JEe6Mo_-4--GvQg" showIcon="false">
           <labelFormat>italic</labelFormat>
         </beginLabelStyle>
-        <centerLabelStyle xmi:type="diagram:CenterLabelStyle" uid="_m-61UtXtEeuF_d0WEhR3Xw" showIcon="false"/>
+        <centerLabelStyle xmi:type="diagram:CenterLabelStyle" uid="_fPrYkr_JEe6Mo_-4--GvQg" showIcon="false"/>
       </ownedStyle>
       <actualMapping xmi:type="description_1:EdgeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@edgeMappings[name='EC%20ESupertypes']"/>
     </ownedDiagramElements>
@@ -1857,32 +1911,40 @@
       </ownedStyle>
       <actualMapping xmi:type="description_1:EdgeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@edgeMappings[name='EC%20ESupertypes']"/>
     </ownedDiagramElements>
-    <ownedDiagramElements xmi:type="diagram:DNodeList" uid="_pcXBkAGyEey7cfH5K6RyCw" name="IndividualDeclaration" tooltipText="" outgoingEdges="_s68oXAGyEey7cfH5K6RyCw _zJpmRgGyEey7cfH5K6RyCw" width="12" height="10">
-      <target xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//IndividualDeclaration"/>
-      <semanticElements xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//IndividualDeclaration"/>
+    <ownedDiagramElements xmi:type="diagram:DNodeList" uid="_pcXBkAGyEey7cfH5K6RyCw" name="NodeDeclaration" tooltipText="" outgoingEdges="_s68oXAGyEey7cfH5K6RyCw _zJpmRgGyEey7cfH5K6RyCw" width="12" height="10">
+      <target xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//NodeDeclaration"/>
+      <semanticElements xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//NodeDeclaration"/>
       <arrangeConstraints>KEEP_LOCATION</arrangeConstraints>
       <arrangeConstraints>KEEP_SIZE</arrangeConstraints>
       <arrangeConstraints>KEEP_RATIO</arrangeConstraints>
-      <ownedStyle xmi:type="diagram:FlatContainerStyle" uid="_pcYPsAGyEey7cfH5K6RyCw" borderSize="1" borderSizeComputationExpression="1" backgroundStyle="Liquid" foregroundColor="255,252,216">
+      <ownedStyle xmi:type="diagram:FlatContainerStyle" uid="_vkEXpL_KEe6Mo_-4--GvQg" borderSize="1" borderSizeComputationExpression="1" backgroundStyle="Liquid" foregroundColor="255,252,216">
         <description xmi:type="style:FlatContainerStyleDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EClass']/@style"/>
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       <actualMapping xmi:type="description_1:ContainerMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EClass']"/>
+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_u4PMYL_KEe6Mo_-4--GvQg" name="kind : NodeKind = NODE" tooltipText="">
+        <target xmi:type="ecore:EAttribute" href="src/main/resources/model/problem.ecore#//NodeDeclaration/kind"/>
+        <semanticElements xmi:type="ecore:EAttribute" href="src/main/resources/model/problem.ecore#//NodeDeclaration/kind"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_vkICAb_KEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EClass']/@subNodeMappings[name='EC%20EAttribute']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EClass']/@subNodeMappings[name='EC%20EAttribute']"/>
+      </ownedElements>
     </ownedDiagramElements>
     <ownedDiagramElements xmi:type="diagram:DEdge" uid="_s68oXAGyEey7cfH5K6RyCw" sourceNode="_pcXBkAGyEey7cfH5K6RyCw" targetNode="_e7ydoKA9EeuqkpDnuik1sg">
-      <target xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//IndividualDeclaration"/>
-      <semanticElements xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//IndividualDeclaration"/>
-      <ownedStyle xmi:type="diagram:EdgeStyle" uid="_s69PYAGyEey7cfH5K6RyCw" lineStyle="dash" targetArrow="InputClosedArrow" routingStyle="tree">
+      <target xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//NodeDeclaration"/>
+      <semanticElements xmi:type="ecore:EClass" href="src/main/resources/model/problem.ecore#//NodeDeclaration"/>
+      <ownedStyle xmi:type="diagram:EdgeStyle" uid="_vkQk5r_KEe6Mo_-4--GvQg" lineStyle="dash" targetArrow="InputClosedArrow" routingStyle="tree">
         <description xmi:type="style:EdgeStyleDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@edgeMappings[name='EC%20ESupertypes']/@conditionnalStyles.0/@style"/>
-        <beginLabelStyle xmi:type="diagram:BeginLabelStyle" uid="_s69PYQGyEey7cfH5K6RyCw" showIcon="false">
+        <beginLabelStyle xmi:type="diagram:BeginLabelStyle" uid="_vkQk57_KEe6Mo_-4--GvQg" showIcon="false">
           <labelFormat>italic</labelFormat>
         </beginLabelStyle>
-        <centerLabelStyle xmi:type="diagram:CenterLabelStyle" uid="_s69PYgGyEey7cfH5K6RyCw" showIcon="false"/>
+        <centerLabelStyle xmi:type="diagram:CenterLabelStyle" uid="_vkQk6L_KEe6Mo_-4--GvQg" showIcon="false"/>
       </ownedStyle>
       <actualMapping xmi:type="description_1:EdgeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@edgeMappings[name='EC%20ESupertypes']"/>
     </ownedDiagramElements>
     <ownedDiagramElements xmi:type="diagram:DEdge" uid="_zJpmRgGyEey7cfH5K6RyCw" name="[0..*] nodes" sourceNode="_pcXBkAGyEey7cfH5K6RyCw" targetNode="_xsYrUKA8EeuqkpDnuik1sg">
-      <target xmi:type="ecore:EReference" href="src/main/resources/model/problem.ecore#//IndividualDeclaration/nodes"/>
-      <semanticElements xmi:type="ecore:EReference" href="src/main/resources/model/problem.ecore#//IndividualDeclaration/nodes"/>
+      <target xmi:type="ecore:EReference" href="src/main/resources/model/problem.ecore#//NodeDeclaration/nodes"/>
+      <semanticElements xmi:type="ecore:EReference" href="src/main/resources/model/problem.ecore#//NodeDeclaration/nodes"/>
       <ownedStyle xmi:type="diagram:EdgeStyle" uid="_zJqNUAGyEey7cfH5K6RyCw" description="_L-JhMKA4EeuqkpDnuik1sg" sourceArrow="FillDiamond" routingStyle="manhattan" strokeColor="0,0,0">
         <centerLabelStyle xmi:type="diagram:CenterLabelStyle" uid="_zJqNUgGyEey7cfH5K6RyCw" showIcon="false">
           <customFeatures>labelSize</customFeatures>
@@ -2560,6 +2622,68 @@
       </ownedStyle>
       <actualMapping xmi:type="description_1:EdgeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@edgeMappings[name='EC_EReference']"/>
     </ownedDiagramElements>
+    <ownedDiagramElements xmi:type="diagram:DNodeList" uid="_ZbwSIL_JEe6Mo_-4--GvQg" name="ModuleKind" tooltipText="" width="12" height="10">
+      <target xmi:type="ecore:EEnum" href="src/main/resources/model/problem.ecore#//ModuleKind"/>
+      <semanticElements xmi:type="ecore:EEnum" href="src/main/resources/model/problem.ecore#//ModuleKind"/>
+      <arrangeConstraints>KEEP_LOCATION</arrangeConstraints>
+      <arrangeConstraints>KEEP_SIZE</arrangeConstraints>
+      <arrangeConstraints>KEEP_RATIO</arrangeConstraints>
+      <ownedStyle xmi:type="diagram:FlatContainerStyle" uid="_Zbw5ML_JEe6Mo_-4--GvQg" borderSize="1" borderSizeComputationExpression="1" borderColor="125,125,125" backgroundStyle="Liquid" foregroundColor="221,236,202">
+        <description xmi:type="style:FlatContainerStyleDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@style"/>
+      </ownedStyle>
+      <actualMapping xmi:type="description_1:ContainerMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']"/>
+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_csN9UL_JEe6Mo_-4--GvQg" name="PROBLEM" tooltipText="">
+        <target xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//ModuleKind/PROBLEM"/>
+        <semanticElements xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//ModuleKind/PROBLEM"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_csN9Ub_JEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']"/>
+      </ownedElements>
+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_dVQTQL_JEe6Mo_-4--GvQg" name="MODULE" tooltipText="">
+        <target xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//ModuleKind/MODULE"/>
+        <semanticElements xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//ModuleKind/MODULE"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_dVQ6UL_JEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']"/>
+      </ownedElements>
+    </ownedDiagramElements>
+    <ownedDiagramElements xmi:type="diagram:DNodeList" uid="_aqfgsL_KEe6Mo_-4--GvQg" name="NodeKind" tooltipText="" width="12" height="10">
+      <target xmi:type="ecore:EEnum" href="src/main/resources/model/problem.ecore#//NodeKind"/>
+      <semanticElements xmi:type="ecore:EEnum" href="src/main/resources/model/problem.ecore#//NodeKind"/>
+      <arrangeConstraints>KEEP_LOCATION</arrangeConstraints>
+      <arrangeConstraints>KEEP_SIZE</arrangeConstraints>
+      <arrangeConstraints>KEEP_RATIO</arrangeConstraints>
+      <ownedStyle xmi:type="diagram:FlatContainerStyle" uid="_aqfgsb_KEe6Mo_-4--GvQg" borderSize="1" borderSizeComputationExpression="1" borderColor="125,125,125" backgroundStyle="Liquid" foregroundColor="221,236,202">
+        <description xmi:type="style:FlatContainerStyleDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@style"/>
+      </ownedStyle>
+      <actualMapping xmi:type="description_1:ContainerMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']"/>
+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_cV-yIL_KEe6Mo_-4--GvQg" name="NODE" tooltipText="">
+        <target xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//NodeKind/NODE"/>
+        <semanticElements xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//NodeKind/NODE"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_cV-yIb_KEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']"/>
+      </ownedElements>
+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_crhkUL_KEe6Mo_-4--GvQg" name="ATOM" tooltipText="">
+        <target xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//NodeKind/ATOM"/>
+        <semanticElements xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//NodeKind/ATOM"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_crhkUb_KEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']"/>
+      </ownedElements>
+      <ownedElements xmi:type="diagram:DNodeListElement" uid="_c7jkYL_KEe6Mo_-4--GvQg" name="MULTI" tooltipText="">
+        <target xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//NodeKind/MULTI"/>
+        <semanticElements xmi:type="ecore:EEnumLiteral" href="src/main/resources/model/problem.ecore#//NodeKind/MULTI"/>
+        <ownedStyle xmi:type="diagram:BundledImage" uid="_c7jkYb_KEe6Mo_-4--GvQg" labelAlignment="LEFT">
+          <description xmi:type="style:BundledImageDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']/@style"/>
+        </ownedStyle>
+        <actualMapping xmi:type="description_1:NodeMapping" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer/@containerMappings[name='EC%20EEnum']/@subNodeMappings[name='EC%20EEnumLiteral']"/>
+      </ownedElements>
+    </ownedDiagramElements>
     <description xmi:type="description_1:DiagramDescription" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']"/>
     <filterVariableHistory xmi:type="diagram:FilterVariableHistory" uid="_CsWlsKA4EeuqkpDnuik1sg"/>
     <activatedLayers xmi:type="description_1:Layer" href="platform:/plugin/org.eclipse.emf.ecoretools.design/description/ecore.odesign#//@ownedViewpoints[name='Design']/@ownedRepresentations[name='Entities']/@defaultLayer"/>
diff --git a/subprojects/language-model/src/main/resources/model/problem.ecore b/subprojects/language-model/src/main/resources/model/problem.ecore
index 74229a89..aa05a2fe 100644
--- a/subprojects/language-model/src/main/resources/model/problem.ecore
+++ b/subprojects/language-model/src/main/resources/model/problem.ecore
@@ -6,6 +6,7 @@
         eType="#//Node" transient="true" containment="true"/>
     <eStructuralFeatures xsi:type="ecore:EReference" name="statements" upperBound="-1"
         eType="#//Statement" containment="true"/>
+    <eStructuralFeatures xsi:type="ecore:EAttribute" name="kind" eType="#//ModuleKind"/>
   </eClassifiers>
   <eClassifiers xsi:type="ecore:EClass" name="ClassDeclaration" eSuperTypes="#//Statement #//Relation">
     <eStructuralFeatures xsi:type="ecore:EAttribute" name="abstract" eType="ecore:EDataType http://www.eclipse.org/emf/2002/Ecore#//EBoolean"/>
@@ -109,9 +110,10 @@
     <eStructuralFeatures xsi:type="ecore:EReference" name="node" eType="#//Node"/>
   </eClassifiers>
   <eClassifiers xsi:type="ecore:EClass" name="AssertionArgument" abstract="true"/>
-  <eClassifiers xsi:type="ecore:EClass" name="IndividualDeclaration" eSuperTypes="#//Statement">
+  <eClassifiers xsi:type="ecore:EClass" name="NodeDeclaration" eSuperTypes="#//Statement">
     <eStructuralFeatures xsi:type="ecore:EReference" name="nodes" upperBound="-1"
         eType="#//Node" containment="true"/>
+    <eStructuralFeatures xsi:type="ecore:EAttribute" name="kind" eType="#//NodeKind"/>
   </eClassifiers>
   <eClassifiers xsi:type="ecore:EClass" name="WildcardAssertionArgument" eSuperTypes="#//AssertionArgument"/>
   <eClassifiers xsi:type="ecore:EClass" name="ParametricDefinition" abstract="true"
@@ -259,4 +261,13 @@
   <eClassifiers xsi:type="ecore:EClass" name="LogicConstant" eSuperTypes="#//Constant">
     <eStructuralFeatures xsi:type="ecore:EAttribute" name="logicValue" eType="#//LogicValue"/>
   </eClassifiers>
+  <eClassifiers xsi:type="ecore:EEnum" name="ModuleKind">
+    <eLiterals name="PROBLEM"/>
+    <eLiterals name="MODULE" value="1"/>
+  </eClassifiers>
+  <eClassifiers xsi:type="ecore:EEnum" name="NodeKind">
+    <eLiterals name="NODE"/>
+    <eLiterals name="ATOM" value="1"/>
+    <eLiterals name="MULTI" value="2"/>
+  </eClassifiers>
 </ecore:EPackage>
diff --git a/subprojects/language-model/src/main/resources/model/problem.genmodel b/subprojects/language-model/src/main/resources/model/problem.genmodel
index b5aa81c4..be8b62eb 100644
--- a/subprojects/language-model/src/main/resources/model/problem.genmodel
+++ b/subprojects/language-model/src/main/resources/model/problem.genmodel
@@ -64,9 +64,19 @@
       <genEnumLiterals ecoreEnumLiteral="problem.ecore#//PrimitiveType/REAL"/>
       <genEnumLiterals ecoreEnumLiteral="problem.ecore#//PrimitiveType/STRING"/>
     </genEnums>
+    <genEnums typeSafeEnumCompatible="false" ecoreEnum="problem.ecore#//ModuleKind">
+      <genEnumLiterals ecoreEnumLiteral="problem.ecore#//ModuleKind/PROBLEM"/>
+      <genEnumLiterals ecoreEnumLiteral="problem.ecore#//ModuleKind/MODULE"/>
+    </genEnums>
+    <genEnums typeSafeEnumCompatible="false" ecoreEnum="problem.ecore#//NodeKind">
+      <genEnumLiterals ecoreEnumLiteral="problem.ecore#//NodeKind/NODE"/>
+      <genEnumLiterals ecoreEnumLiteral="problem.ecore#//NodeKind/ATOM"/>
+      <genEnumLiterals ecoreEnumLiteral="problem.ecore#//NodeKind/MULTI"/>
+    </genEnums>
     <genClasses ecoreClass="problem.ecore#//Problem">
       <genFeatures property="None" children="true" createChild="true" ecoreFeature="ecore:EReference problem.ecore#//Problem/nodes"/>
       <genFeatures property="None" children="true" createChild="true" ecoreFeature="ecore:EReference problem.ecore#//Problem/statements"/>
+      <genFeatures createChild="false" ecoreFeature="ecore:EAttribute problem.ecore#//Problem/kind"/>
     </genClasses>
     <genClasses ecoreClass="problem.ecore#//ClassDeclaration">
       <genFeatures createChild="false" ecoreFeature="ecore:EAttribute problem.ecore#//ClassDeclaration/abstract"/>
@@ -144,8 +154,9 @@
       <genFeatures notify="false" createChild="false" propertySortChoices="true" ecoreFeature="ecore:EReference problem.ecore#//NodeAssertionArgument/node"/>
     </genClasses>
     <genClasses ecoreClass="problem.ecore#//AssertionArgument"/>
-    <genClasses ecoreClass="problem.ecore#//IndividualDeclaration">
-      <genFeatures property="None" children="true" createChild="true" ecoreFeature="ecore:EReference problem.ecore#//IndividualDeclaration/nodes"/>
+    <genClasses ecoreClass="problem.ecore#//NodeDeclaration">
+      <genFeatures property="None" children="true" createChild="true" ecoreFeature="ecore:EReference problem.ecore#//NodeDeclaration/nodes"/>
+      <genFeatures createChild="false" ecoreFeature="ecore:EAttribute problem.ecore#//NodeDeclaration/kind"/>
     </genClasses>
     <genClasses ecoreClass="problem.ecore#//WildcardAssertionArgument"/>
     <genClasses ecoreClass="problem.ecore#//ParametricDefinition">
diff --git a/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java b/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java
index 38bf5a61..b462fd70 100644
--- a/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java
+++ b/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/ModelInitializer.java
@@ -169,9 +169,9 @@ public class ModelInitializer {
 
         private void collectNodes() {
                 for (var statement : problem.getStatements()) {
-                        if (statement instanceof IndividualDeclaration individualDeclaration) {
-                                for (var individual : individualDeclaration.getNodes()) {
-                                        collectNode(individual);
+                        if (statement instanceof NodeDeclaration nodeDeclaration) {
+                                for (var node : nodeDeclaration.getNodes()) {
+                                        collectNode(node);
                                 }
                         } else if (statement instanceof ClassDeclaration classDeclaration) {
                                 var newNode = classDeclaration.getNewNode();
@@ -355,10 +355,8 @@ public class ModelInitializer {
                                 collectClassDeclarationAssertions(classDeclaration);
                         } else if (statement instanceof EnumDeclaration enumDeclaration) {
                                 collectEnumAssertions(enumDeclaration);
-                        } else if (statement instanceof IndividualDeclaration individualDeclaration) {
-                                for (var individual : individualDeclaration.getNodes()) {
-                                        collectIndividualAssertions(individual);
-                                }
+                        } else if (statement instanceof NodeDeclaration nodeDeclaration) {
+                                collectNodeDeclarationAssertions(nodeDeclaration);
                         } else if (statement instanceof Assertion assertion) {
                                 collectAssertion(assertion);
                         }
@@ -379,7 +377,7 @@ public class ModelInitializer {
         private void collectEnumAssertions(EnumDeclaration enumDeclaration) {
                 var overlay = MutableSeed.of(1, null);
                 for (var literal : enumDeclaration.getLiterals()) {
-                        collectIndividualAssertions(literal);
+                        collectCardinalityAssertions(literal, TruthValue.TRUE);
                         var nodeId = getNodeId(literal);
                         overlay.mergeValue(Tuple.of(nodeId), TruthValue.TRUE);
                 }
@@ -387,9 +385,25 @@ public class ModelInitializer {
                 info.assertions().overwriteValues(overlay);
         }
 
-        private void collectIndividualAssertions(Node node) {
+        private void collectNodeDeclarationAssertions(NodeDeclaration nodeDeclaration) {
+                var kind = nodeDeclaration.getKind();
+                TruthValue value;
+                switch (kind) {
+                case ATOM -> value = TruthValue.TRUE;
+                case MULTI -> value = TruthValue.UNKNOWN;
+                case NODE -> {
+                        return;
+                }
+                default -> throw new IllegalArgumentException("Unknown node kind: " + kind);
+                }
+                for (var node : nodeDeclaration.getNodes()) {
+                        collectCardinalityAssertions(node, value);
+                }
+        }
+
+        private void collectCardinalityAssertions(Node node, TruthValue value) {
                 var nodeId = getNodeId(node);
-                collectCardinalityAssertions(nodeId, TruthValue.TRUE);
+                collectCardinalityAssertions(nodeId, value);
         }
 
         private void collectCardinalityAssertions(int nodeId, TruthValue value) {
@@ -587,47 +601,51 @@ public class ModelInitializer {
 
         private void toLiterals(Expr expr, Map<tools.refinery.language.model.problem.Variable, Variable> localScope,
                                                         List<Literal> literals) {
-                if (expr instanceof LogicConstant logicConstant) {
-                        switch (logicConstant.getLogicValue()) {
-                        case TRUE -> literals.add(BooleanLiteral.TRUE);
-                        case FALSE -> literals.add(BooleanLiteral.FALSE);
-                        default -> throw new TracedException(logicConstant, "Unsupported literal");
-                        }
-                } else if (expr instanceof Atom atom) {
-                        var target = getPartialRelation(atom.getRelation());
-                        var polarity = atom.isTransitiveClosure() ? CallPolarity.TRANSITIVE : CallPolarity.POSITIVE;
-                        var argumentList = toArgumentList(atom.getArguments(), localScope, literals);
-                        literals.add(target.call(polarity, argumentList));
-                } else if (expr instanceof NegationExpr negationExpr) {
-                        var body = negationExpr.getBody();
-                        if (!(body instanceof Atom atom)) {
-                                throw new TracedException(body, "Cannot negate literal");
-                        }
-                        var target = getPartialRelation(atom.getRelation());
-                        Constraint constraint;
-                        if (atom.isTransitiveClosure()) {
-                                constraint = Query.of(target.name() + "#transitive", (builder, p1, p2) -> builder.clause(
-                                                target.callTransitive(p1, p2)
-                                )).getDnf();
-                        } else {
-                                constraint = target;
-                        }
-                        var negatedScope = extendScope(localScope, negationExpr.getImplicitVariables());
-                        var argumentList = toArgumentList(atom.getArguments(), negatedScope, literals);
-                        literals.add(constraint.call(CallPolarity.NEGATIVE, argumentList));
-                } else if (expr instanceof ComparisonExpr comparisonExpr) {
-                        var argumentList = toArgumentList(List.of(comparisonExpr.getLeft(), comparisonExpr.getRight()),
-                                        localScope, literals);
-                        boolean positive = switch (comparisonExpr.getOp()) {
-                                case EQ -> true;
-                                case NOT_EQ -> false;
-                                default -> throw new TracedException(
-                                                comparisonExpr, "Unsupported operator");
-                        };
-                        literals.add(new EquivalenceLiteral(positive, argumentList.get(0), argumentList.get(1)));
-                } else {
-                        throw new TracedException(expr, "Unsupported literal");
-                }
+        switch (expr) {
+            case LogicConstant logicConstant -> {
+                switch (logicConstant.getLogicValue()) {
+                    case TRUE -> literals.add(BooleanLiteral.TRUE);
+                    case FALSE -> literals.add(BooleanLiteral.FALSE);
+                    default -> throw new TracedException(logicConstant, "Unsupported literal");
+                }
+            }
+            case Atom atom -> {
+                var target = getPartialRelation(atom.getRelation());
+                var polarity = atom.isTransitiveClosure() ? CallPolarity.TRANSITIVE : CallPolarity.POSITIVE;
+                var argumentList = toArgumentList(atom.getArguments(), localScope, literals);
+                literals.add(target.call(polarity, argumentList));
+            }
+            case NegationExpr negationExpr -> {
+                var body = negationExpr.getBody();
+                if (!(body instanceof Atom atom)) {
+                    throw new TracedException(body, "Cannot negate literal");
+                }
+                var target = getPartialRelation(atom.getRelation());
+                Constraint constraint;
+                if (atom.isTransitiveClosure()) {
+                    constraint = Query.of(target.name() + "#transitive", (builder, p1, p2) -> builder.clause(
+                            target.callTransitive(p1, p2)
+                    )).getDnf();
+                } else {
+                    constraint = target;
+                }
+                var negatedScope = extendScope(localScope, negationExpr.getImplicitVariables());
+                var argumentList = toArgumentList(atom.getArguments(), negatedScope, literals);
+                literals.add(constraint.call(CallPolarity.NEGATIVE, argumentList));
+            }
+            case ComparisonExpr comparisonExpr -> {
+                var argumentList = toArgumentList(List.of(comparisonExpr.getLeft(), comparisonExpr.getRight()),
+                        localScope, literals);
+                boolean positive = switch (comparisonExpr.getOp()) {
+                    case EQ -> true;
+                    case NOT_EQ -> false;
+                    default -> throw new TracedException(
+                            comparisonExpr, "Unsupported operator");
+                };
+                literals.add(new EquivalenceLiteral(positive, argumentList.get(0), argumentList.get(1)));
+            }
+            default -> throw new TracedException(expr, "Unsupported literal");
+        }
         }
 
         private List<Variable> toArgumentList(
diff --git a/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/SolutionSerializer.java b/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/SolutionSerializer.java
index 09ba34fc..537d94ca 100644
--- a/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/SolutionSerializer.java
+++ b/subprojects/language-semantics/src/main/java/tools/refinery/language/semantics/SolutionSerializer.java
@@ -197,9 +197,10 @@ public class SolutionSerializer {
                         var originalNode = pair.getOne();
                         int nodeId = pair.getTwo();
                         var newNode = findNode(originalNode);
-                        // Since all implicit nodes that do not exist has already been remove in serializeSolution,
-                        // we only need to add !exists assertions to ::new nodes (nodes marked as an individual must always exist).
-                        if (ProblemUtil.isNewNode(originalNode)) {
+                        // Since all implicit nodes that do not exist has already been removed in serializeSolution,
+                        // we only need to add !exists assertions to ::new nodes and explicitly declared nodes that do not exist.
+                        if (ProblemUtil.isMultiNode(originalNode) ||
+                                        (ProblemUtil.isDeclaredNode(originalNode) && !isExistingNode(nodeId))) {
                                 sortedNewNodes.put(nodeId, newNode);
                         } else {
                                 nodes.put(nodeId, newNode);
diff --git a/subprojects/language-semantics/src/test/java/tools/refinery/language/semantics/SolutionSerializerTest.java b/subprojects/language-semantics/src/test/java/tools/refinery/language/semantics/SolutionSerializerTest.java
index 8ef449b0..b682a7d6 100644
--- a/subprojects/language-semantics/src/test/java/tools/refinery/language/semantics/SolutionSerializerTest.java
+++ b/subprojects/language-semantics/src/test/java/tools/refinery/language/semantics/SolutionSerializerTest.java
@@ -183,6 +183,49 @@ class SolutionSerializerTest {
                                 !exists(Foo::new).
                                 Foo(a).
                                 default !ref(*, *).
+                                """), Arguments.of("""
+                                atom a.
+                                class Foo.
+                                """, """
+                                Foo(a).
+                                scope Foo += 0.
+                                """, """
+                                !exists(Foo::new).
+                                Foo(a).
+                                """), Arguments.of("""
+                                multi a.
+                                class Foo.
+                                """, """
+                                Foo(a).
+                                !exists(Foo::new).
+                                scope Foo = 2.
+                                """, """
+                                !exists(a).
+                                !exists(Foo::new).
+                                Foo(foo1).
+                                Foo(foo2).
+                                """), Arguments.of("""
+                                node a.
+                                class Foo.
+                                """, """
+                                Foo(a).
+                                ?exists(a).
+                                scope Foo = 2, Foo += 1.
+                                """, """
+                                !exists(Foo::new).
+                                Foo(a).
+                                Foo(foo1).
+                                """), Arguments.of("""
+                                node a.
+                                class Foo.
+                                """, """
+                                Foo(a).
+                                ?exists(a).
+                                scope Foo = 1, Foo += 1.
+                                """, """
+                                !exists(a).
+                                !exists(Foo::new).
+                                Foo(foo1).
                                 """));
         }
 }
diff --git a/subprojects/language-web/src/main/java/tools/refinery/language/web/semantics/metadata/MetadataCreator.java b/subprojects/language-web/src/main/java/tools/refinery/language/web/semantics/metadata/MetadataCreator.java
index f05abc45..1f49b0d0 100644
--- a/subprojects/language-web/src/main/java/tools/refinery/language/web/semantics/metadata/MetadataCreator.java
+++ b/subprojects/language-web/src/main/java/tools/refinery/language/web/semantics/metadata/MetadataCreator.java
@@ -87,14 +87,12 @@ public class MetadataCreator {
         }
 
         private NodeKind getNodeKind(Node node) {
-                if (ProblemUtil.isImplicitNode(node)) {
-                        return NodeKind.IMPLICIT;
-                } else if (ProblemUtil.isIndividualNode(node)) {
+                if (ProblemUtil.isAtomNode(node)) {
                         return NodeKind.INDIVIDUAL;
-                } else if (ProblemUtil.isNewNode(node)) {
+                } else if (ProblemUtil.isMultiNode(node)) {
                         return NodeKind.NEW;
                 } else {
-                        throw new TracedException(node, "Unknown node type");
+                        return NodeKind.IMPLICIT;
                 }
         }
 
diff --git a/subprojects/language-web/src/test/java/tools/refinery/language/web/ProblemWebSocketServletIntegrationTest.java b/subprojects/language-web/src/test/java/tools/refinery/language/web/ProblemWebSocketServletIntegrationTest.java
index 889a55cb..dd5d8a03 100644
--- a/subprojects/language-web/src/test/java/tools/refinery/language/web/ProblemWebSocketServletIntegrationTest.java
+++ b/subprojects/language-web/src/test/java/tools/refinery/language/web/ProblemWebSocketServletIntegrationTest.java
@@ -129,7 +129,7 @@ class ProblemWebSocketServletIntegrationTest {
                         case 4 -> //noinspection TextBlockMigration
                                         session.sendText(
                                         "{\"id\":\"bar\",\"request\":{\"resource\":\"test.problem\",\"serviceType\":\"update\"," +
-                                                        "\"requiredStateId\":\"-80000000\",\"deltaText\":\"indiv q.\nnode(q).\n\"," +
+                                                        "\"requiredStateId\":\"-80000000\",\"deltaText\":\"atom q.\nnode(q).\n\"," +
                                                         "\"deltaOffset\":\"0\",\"deltaReplaceLength\":\"0\"}}",
                                         Callback.NOOP
                         );
diff --git a/subprojects/language-web/src/test/java/tools/refinery/language/web/xtext/servlet/TransactionExecutorTest.java b/subprojects/language-web/src/test/java/tools/refinery/language/web/xtext/servlet/TransactionExecutorTest.java
index 22ce1b47..265723ff 100644
--- a/subprojects/language-web/src/test/java/tools/refinery/language/web/xtext/servlet/TransactionExecutorTest.java
+++ b/subprojects/language-web/src/test/java/tools/refinery/language/web/xtext/servlet/TransactionExecutorTest.java
@@ -97,7 +97,7 @@ class TransactionExecutorTest {
                 var stateId = updateFullText();
                 var responseHandler = sendRequestAndWaitForAllResponses(
                                 new XtextWebRequest("bar", Map.of("resource", RESOURCE_NAME, "serviceType", "update", "requiredStateId",
-                                                stateId, "deltaText", "indiv q.\nnode(q).\n", "deltaOffset", "0", "deltaReplaceLength",
+                                                stateId, "deltaText", "atom q.\nnode(q).\n", "deltaOffset", "0", "deltaReplaceLength",
                                                 "0")));
 
                 var captor = newCaptor();
diff --git a/subprojects/language/src/main/java/tools/refinery/language/GenerateProblem.mwe2 b/subprojects/language/src/main/java/tools/refinery/language/GenerateProblem.mwe2
index 59eba8f7..863e55d4 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/GenerateProblem.mwe2
+++ b/subprojects/language/src/main/java/tools/refinery/language/GenerateProblem.mwe2
@@ -51,7 +51,7 @@ Workflow {
 
                 language = StandardLanguage {
                         name = 'tools.refinery.language.Problem'
-                        fileExtensions = 'problem'
+                        fileExtensions = 'refinery,problem'
                         referencedResource = 'platform:/resource/tools.refinery.refinery-language-model/model/problem.genmodel'
                         serializer = {
                                 generateStub = false
diff --git a/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext b/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
index 0fb96954..c5c42ebe 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
+++ b/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
@@ -9,13 +9,16 @@ import "http://www.eclipse.org/emf/2002/Ecore" as ecore
 import "https://refinery.tools/emf/2021/Problem"
 
 Problem:
-        ("problem" name=Identifier ".")?
+        (kind=ModuleKind name=QualifiedName? ".")?
         statements+=Statement*;
 
+enum ModuleKind:
+        PROBLEM="problem" | MODULE="module";
+
 Statement:
         Assertion | ClassDeclaration | EnumDeclaration |
         PredicateDefinition | /* FunctionDefinition | RuleDefinition | */
-        ScopeDeclaration | IndividualDeclaration;
+        ScopeDeclaration | NodeDeclaration;
 
 ClassDeclaration:
         abstract?="abstract"? "class"
@@ -252,8 +255,11 @@ RangeMultiplicity:
 ExactMultiplicity:
         exactValue=INT;
 
-IndividualDeclaration:
-        "indiv" nodes+=EnumLiteral ("," nodes+=EnumLiteral)* ".";
+NodeDeclaration:
+        kind=NodeKind nodes+=EnumLiteral ("," nodes+=EnumLiteral)* ".";
+
+enum NodeKind:
+        NODE="node" | ATOM="atom" | MULTI="multi";
 
 UpperBound returns ecore::EInt:
         INT | "*";
@@ -268,7 +274,8 @@ Identifier:
         NonContainmentIdentifier | "contains" | "container";
 
 NonContainmentIdentifier:
-        ID | "contained" | "sum" | "prod" | "min" | "max";
+        ID | "node" | "atom" | "multi" | "contained" |
+        "sum" | "prod" | "min" | "max" | "problem" | "module";
 
 Real returns ecore::EDouble:
         EXPONENTIAL | INT "." (INT | EXPONENTIAL);
diff --git a/subprojects/language/src/main/java/tools/refinery/language/formatting2/ProblemFormatter.java b/subprojects/language/src/main/java/tools/refinery/language/formatting2/ProblemFormatter.java
index 0f3bd3ee..0b87e8bb 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/formatting2/ProblemFormatter.java
+++ b/subprojects/language/src/main/java/tools/refinery/language/formatting2/ProblemFormatter.java
@@ -23,7 +23,7 @@ public class ProblemFormatter extends AbstractJavaFormatter {
         protected void format(Problem problem, IFormattableDocument doc) {
                 doc.prepend(problem, this::noSpace);
                 var region = regionFor(problem);
-                doc.append(region.keyword("problem"), this::oneSpace);
+                doc.append(region.feature(ProblemPackage.Literals.PROBLEM__KIND), this::oneSpace);
                 doc.prepend(region.keyword("."), this::noSpace);
                 appendNewLines(doc, region.keyword("."), this::twoNewLines);
                 for (var statement : problem.getStatements()) {
@@ -132,10 +132,10 @@ public class ProblemFormatter extends AbstractJavaFormatter {
                 }
         }
 
-        protected void format(IndividualDeclaration individualDeclaration, IFormattableDocument doc) {
-                surroundNewLines(doc, individualDeclaration, this::singleNewLine);
-                var region = regionFor(individualDeclaration);
-                doc.append(region.keyword("indiv"), this::oneSpace);
+        protected void format(NodeDeclaration nodeDeclaration, IFormattableDocument doc) {
+                surroundNewLines(doc, nodeDeclaration, this::singleNewLine);
+                var region = regionFor(nodeDeclaration);
+                doc.append(region.feature(ProblemPackage.Literals.NODE_DECLARATION__KIND), this::oneSpace);
                 formatList(region, ",", doc);
                 doc.prepend(region.keyword("."), this::noSpace);
         }
diff --git a/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemLocationInFileProvider.java b/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemLocationInFileProvider.java
index 29eaad84..c81431e5 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemLocationInFileProvider.java
+++ b/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemLocationInFileProvider.java
@@ -25,10 +25,10 @@ public class ProblemLocationInFileProvider extends DefaultLocationInFileProvider
         }
 
         protected ITextRegion getNodeTextRegion(Node node, RegionDescription query) {
-                if (ProblemUtil.isIndividualNode(node)) {
+                if (ProblemUtil.isDeclaredNode(node)) {
                         return super.doGetTextRegion(node, query);
                 }
-                if (ProblemUtil.isNewNode(node)) {
+                if (ProblemUtil.isMultiNode(node)) {
                         EObject container = node.eContainer();
                         return doGetTextRegion(container, query);
                 }
diff --git a/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemResourceDescriptionStrategy.java b/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemResourceDescriptionStrategy.java
index c04c7d09..79dad6e7 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemResourceDescriptionStrategy.java
+++ b/subprojects/language/src/main/java/tools/refinery/language/resource/ProblemResourceDescriptionStrategy.java
@@ -91,8 +91,7 @@ public class ProblemResourceDescriptionStrategy extends DefaultResourceDescripti
                         return false;
                 }
                 if (eObject instanceof Node node) {
-                        // Only enum literals and new nodes are visible across problem files.
-                        return ProblemUtil.isIndividualNode(node) || ProblemUtil.isNewNode(node);
+                        return !ProblemUtil.isImplicitNode(node);
                 }
                 return true;
         }
@@ -111,7 +110,7 @@ public class ProblemResourceDescriptionStrategy extends DefaultResourceDescripti
 
         protected boolean shouldExportSimpleName(EObject eObject) {
                 if (eObject instanceof Node node) {
-                        return !ProblemUtil.isNewNode(node);
+                        return !ProblemUtil.isMultiNode(node);
                 }
                 if (eObject instanceof PredicateDefinition predicateDefinition) {
                         return !ProblemUtil.isInvalidMultiplicityConstraint(predicateDefinition);
@@ -146,6 +145,5 @@ public class ProblemResourceDescriptionStrategy extends DefaultResourceDescripti
                         return false;
                 }
                 return eObject instanceof ClassDeclaration || eObject instanceof EnumDeclaration;
-
         }
 }
diff --git a/subprojects/language/src/main/java/tools/refinery/language/utils/ProblemUtil.java b/subprojects/language/src/main/java/tools/refinery/language/utils/ProblemUtil.java
index 7b6407e1..ee0e141d 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/utils/ProblemUtil.java
+++ b/subprojects/language/src/main/java/tools/refinery/language/utils/ProblemUtil.java
@@ -54,14 +54,24 @@ public final class ProblemUtil {
                 return eObject instanceof PredicateDefinition predicateDefinition && predicateDefinition.isError();
         }
 
-        public static boolean isIndividualNode(Node node) {
+        public static boolean isAtomNode(Node node) {
                 var containingFeature = node.eContainingFeature();
-                return containingFeature == ProblemPackage.Literals.INDIVIDUAL_DECLARATION__NODES
-                                || containingFeature == ProblemPackage.Literals.ENUM_DECLARATION__LITERALS;
+                if (containingFeature == ProblemPackage.Literals.NODE_DECLARATION__NODES) {
+                        return ((NodeDeclaration) node.eContainer()).getKind() == NodeKind.ATOM;
+                }
+                return containingFeature == ProblemPackage.Literals.ENUM_DECLARATION__LITERALS;
+        }
+
+        public static boolean isMultiNode(Node node) {
+                var containingFeature = node.eContainingFeature();
+                if (containingFeature == ProblemPackage.Literals.NODE_DECLARATION__NODES) {
+                        return ((NodeDeclaration) node.eContainer()).getKind() == NodeKind.MULTI;
+                }
+                return containingFeature == ProblemPackage.Literals.CLASS_DECLARATION__NEW_NODE;
         }
 
-        public static boolean isNewNode(Node node) {
-                return node.eContainingFeature() == ProblemPackage.Literals.CLASS_DECLARATION__NEW_NODE;
+        public static boolean isDeclaredNode(Node node) {
+                return node.eContainingFeature() == ProblemPackage.Literals.NODE_DECLARATION__NODES;
         }
 
         public static boolean isInvalidMultiplicityConstraint(PredicateDefinition predicateDefinition) {
diff --git a/subprojects/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java b/subprojects/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java
index 8bda4b95..a95e8b3c 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java
+++ b/subprojects/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java
@@ -84,7 +84,7 @@ public class ProblemValidator extends AbstractProblemValidator {
         @Check
         public void checkNodeConstants(VariableOrNodeExpr expr) {
                 var variableOrNode = expr.getVariableOrNode();
-                if (variableOrNode instanceof Node node && !ProblemUtil.isIndividualNode(node)) {
+                if (variableOrNode instanceof Node node && !ProblemUtil.isAtomNode(node)) {
                         var name = node.getName();
                         var message = ("Only individuals can be referenced in predicates. " +
                                         "Mark '%s' as individual with the declaration 'indiv %s.'").formatted(name, name);
@@ -96,16 +96,16 @@ public class ProblemValidator extends AbstractProblemValidator {
         @Check
         public void checkUniqueDeclarations(Problem problem) {
                 var relations = new ArrayList<Relation>();
-                var individuals = new ArrayList<Node>();
+                var nodes = new ArrayList<Node>();
                 for (var statement : problem.getStatements()) {
                         if (statement instanceof Relation relation) {
                                 relations.add(relation);
-                        } else if (statement instanceof IndividualDeclaration individualDeclaration) {
-                                individuals.addAll(individualDeclaration.getNodes());
+                        } else if (statement instanceof NodeDeclaration nodeDeclaration) {
+                                nodes.addAll(nodeDeclaration.getNodes());
                         }
                 }
                 checkUniqueSimpleNames(relations);
-                checkUniqueSimpleNames(individuals);
+                checkUniqueSimpleNames(nodes);
         }
 
         @Check
@@ -362,7 +362,7 @@ public class ProblemValidator extends AbstractProblemValidator {
                         return;
                 }
                 var node = getNodeArgumentForMultiObjectAssertion(arguments.get(0));
-                if (node != null && !node.eIsProxy() && ProblemUtil.isIndividualNode(node)) {
+                if (node != null && !node.eIsProxy() && ProblemUtil.isAtomNode(node)) {
                         acceptError("Individual nodes must exist.", assertion, null, 0, UNSUPPORTED_ASSERTION_ISSUE);
                 }
         }
diff --git a/subprojects/language/src/test/java/tools/refinery/language/tests/ProblemParsingTest.java b/subprojects/language/src/test/java/tools/refinery/language/tests/ProblemParsingTest.java
index 72d57f54..17ae5fbb 100644
--- a/subprojects/language/src/test/java/tools/refinery/language/tests/ProblemParsingTest.java
+++ b/subprojects/language/src/test/java/tools/refinery/language/tests/ProblemParsingTest.java
@@ -42,7 +42,7 @@ class ProblemParsingTest {
                                 error invalidTaxStatus(Person p) <->
                                         taxStatus(p, CHILD), children(p, _q).
 
-                                indiv family.
+                                atom family.
                                 Family(family).
                                 members(family, anne): true.
                                 members(family, bob).
diff --git a/subprojects/language/src/test/java/tools/refinery/language/tests/formatting2/ProblemFormatterTest.java b/subprojects/language/src/test/java/tools/refinery/language/tests/formatting2/ProblemFormatterTest.java
index f688d970..4a15f9de 100644
--- a/subprojects/language/src/test/java/tools/refinery/language/tests/formatting2/ProblemFormatterTest.java
+++ b/subprojects/language/src/test/java/tools/refinery/language/tests/formatting2/ProblemFormatterTest.java
@@ -183,8 +183,8 @@ class ProblemFormatterTest {
         }
 
         @Test
-        void individualDeclarationTest() {
-                testFormatter("  indiv  a  ,  b  .  ", "indiv a, b.\n");
+        void atomDeclarationTest() {
+                testFormatter("  atom  a  ,  b  .  ", "atom a, b.\n");
         }
 
         @Test
@@ -194,7 +194,7 @@ class ProblemFormatterTest {
                                 pred foo(node a).
                                 class Foo.
                                 foo(n1, n2).
-                                indiv i1.
+                                atom i1.
                                 !foo(i1, n1).
                                 pred bar(node a, node b).
                                 pred quux().
@@ -207,7 +207,7 @@ class ProblemFormatterTest {
                                 class Foo.
 
                                 foo(n1, n2).
-                                indiv i1.
+                                atom i1.
                                 !foo(i1, n1).
 
                                 pred bar(node a, node b).
diff --git a/subprojects/language/src/test/java/tools/refinery/language/tests/scoping/NodeScopingTest.java b/subprojects/language/src/test/java/tools/refinery/language/tests/scoping/NodeScopingTest.java
index e76d2993..bc0320a6 100644
--- a/subprojects/language/src/test/java/tools/refinery/language/tests/scoping/NodeScopingTest.java
+++ b/subprojects/language/src/test/java/tools/refinery/language/tests/scoping/NodeScopingTest.java
@@ -69,35 +69,35 @@ class NodeScopingTest {
         }
 
         @ParameterizedTest
-        @MethodSource("individualNodeReferenceSource")
-        void individualNodeInAssertionTest(String qualifiedNamePrefix, boolean namedProblem) {
+        @MethodSource("atomNodeReferenceSource")
+        void atomNodeInAssertionTest(String qualifiedNamePrefix, boolean namedProblem) {
                 var problem = parse("""
-                                indiv a, b.
+                                atom a, b.
                                 pred predicate(node x, node y) <-> node(x).
                                 predicate({PARAM}a, {PARAM}a).
                                 ?predicate({PARAM}a, {PARAM}b).
                                 """, qualifiedNamePrefix, namedProblem);
                 assertThat(problem.getResourceErrors(), empty());
                 assertThat(problem.nodeNames(), empty());
-                assertThat(problem.assertion(0).arg(0).node(), equalTo(problem.individualNode("a")));
-                assertThat(problem.assertion(0).arg(1).node(), equalTo(problem.individualNode("a")));
-                assertThat(problem.assertion(1).arg(0).node(), equalTo(problem.individualNode("a")));
-                assertThat(problem.assertion(1).arg(1).node(), equalTo(problem.individualNode("b")));
+                assertThat(problem.assertion(0).arg(0).node(), equalTo(problem.atomNode("a")));
+                assertThat(problem.assertion(0).arg(1).node(), equalTo(problem.atomNode("a")));
+                assertThat(problem.assertion(1).arg(0).node(), equalTo(problem.atomNode("a")));
+                assertThat(problem.assertion(1).arg(1).node(), equalTo(problem.atomNode("b")));
         }
 
         @ParameterizedTest
-        @MethodSource("individualNodeReferenceSource")
-        void individualNodeInPredicateTest(String qualifiedNamePrefix, boolean namedProblem) {
+        @MethodSource("atomNodeReferenceSource")
+        void atomNodeInPredicateTest(String qualifiedNamePrefix, boolean namedProblem) {
                 var problem = parse("""
-                                indiv b.
+                                atom b.
                                 pred predicate(node a) <-> node({PARAM}b).
-                                """);
+                                """, qualifiedNamePrefix, namedProblem);
                 assertThat(problem.getResourceErrors(), empty());
                 assertThat(problem.nodeNames(), empty());
-                assertThat(problem.pred("predicate").conj(0).lit(0).arg(0).node(), equalTo(problem.individualNode("b")));
+                assertThat(problem.pred("predicate").conj(0).lit(0).arg(0).node(), equalTo(problem.atomNode("b")));
         }
 
-        static Stream<Arguments> individualNodeReferenceSource() {
+        static Stream<Arguments> atomNodeReferenceSource() {
                 return Stream.of(Arguments.of("", false), Arguments.of("", true), Arguments.of("test::", true));
         }
 
diff --git a/subprojects/language/src/test/java/tools/refinery/language/tests/serializer/ProblemSerializerTest.java b/subprojects/language/src/test/java/tools/refinery/language/tests/serializer/ProblemSerializerTest.java
index 65675b6b..ad583f8e 100644
--- a/subprojects/language/src/test/java/tools/refinery/language/tests/serializer/ProblemSerializerTest.java
+++ b/subprojects/language/src/test/java/tools/refinery/language/tests/serializer/ProblemSerializerTest.java
@@ -56,15 +56,16 @@ class ProblemSerializerTest {
                 var pred = createPred();
                 var node = ProblemFactory.eINSTANCE.createNode();
                 node.setName("a");
-                var individualDeclaration = ProblemFactory.eINSTANCE.createIndividualDeclaration();
-                individualDeclaration.getNodes().add(node);
-                problem.getStatements().add(individualDeclaration);
+                var atomDeclaration = ProblemFactory.eINSTANCE.createNodeDeclaration();
+                atomDeclaration.setKind(NodeKind.ATOM);
+                atomDeclaration.getNodes().add(node);
+                problem.getStatements().add(atomDeclaration);
                 createAssertion(pred, node, value);
 
                 assertSerializedResult("""
                                 pred foo(node p).
 
-                                indiv a.
+                                atom a.
                                 """ + serializedAssertion + "\n");
         }
 
@@ -79,15 +80,16 @@ class ProblemSerializerTest {
                 var pred = createPred();
                 var node = ProblemFactory.eINSTANCE.createNode();
                 node.setName("a");
-                var individualDeclaration = ProblemFactory.eINSTANCE.createIndividualDeclaration();
-                individualDeclaration.getNodes().add(node);
-                problem.getStatements().add(individualDeclaration);
+                var atomDeclaration = ProblemFactory.eINSTANCE.createNodeDeclaration();
+                atomDeclaration.setKind(NodeKind.ATOM);
+                atomDeclaration.getNodes().add(node);
+                problem.getStatements().add(atomDeclaration);
                 createAssertion(pred, node, value, true);
 
                 assertSerializedResult("""
                                 pred foo(node p).
 
-                                indiv a.
+                                atom a.
                                 default\040""" + serializedAssertion + "\n");
         }
 
diff --git a/subprojects/language/src/testFixtures/java/tools/refinery/language/model/tests/utils/WrappedProblem.java b/subprojects/language/src/testFixtures/java/tools/refinery/language/model/tests/utils/WrappedProblem.java
index fc51ff57..58bfce44 100644
--- a/subprojects/language/src/testFixtures/java/tools/refinery/language/model/tests/utils/WrappedProblem.java
+++ b/subprojects/language/src/testFixtures/java/tools/refinery/language/model/tests/utils/WrappedProblem.java
@@ -67,8 +67,9 @@ public record WrappedProblem(Problem problem) {
                 return ProblemNavigationUtil.named(problem.getNodes(), name);
         }
 
-        public Node individualNode(String name) {
-                var uniqueNodes = statementsOfType(IndividualDeclaration.class)
+        public Node atomNode(String name) {
+                var uniqueNodes = statementsOfType(NodeDeclaration.class)
+                                .filter(declaration -> declaration.getKind() == NodeKind.ATOM)
                                 .flatMap(declaration -> declaration.getNodes().stream());
                 return ProblemNavigationUtil.named(uniqueNodes, name);
         }