Merge pull request #10 from kris7t/cm6-fixes

More CodeMirrror fixes

13 files changed, 302 insertions, 41 deletions

diff --git a/language-ide/src/main/java/tools/refinery/language/ide/ProblemIdeModule.java b/language-ide/src/main/java/tools/refinery/language/ide/ProblemIdeModule.java
index 3502c29f..51cecf06 100644
--- a/language-ide/src/main/java/tools/refinery/language/ide/ProblemIdeModule.java
+++ b/language-ide/src/main/java/tools/refinery/language/ide/ProblemIdeModule.java
@@ -4,9 +4,11 @@
 package tools.refinery.language.ide;
 
 import org.eclipse.xtext.ide.editor.contentassist.IPrefixMatcher;
+import org.eclipse.xtext.ide.editor.contentassist.IdeCrossrefProposalProvider;
 import org.eclipse.xtext.ide.editor.syntaxcoloring.ISemanticHighlightingCalculator;
 
 import tools.refinery.language.ide.contentassist.FuzzyMatcher;
+import tools.refinery.language.ide.contentassist.ProblemCrossrefProposalProvider;
 import tools.refinery.language.ide.syntaxcoloring.ProblemSemanticHighlightingCalculator;
 
 /**
@@ -16,9 +18,13 @@ public class ProblemIdeModule extends AbstractProblemIdeModule {
         public Class<? extends ISemanticHighlightingCalculator> bindISemanticHighlightingCalculator() {
                 return ProblemSemanticHighlightingCalculator.class;
         }
-        
+
         @Override
         public Class<? extends IPrefixMatcher> bindIPrefixMatcher() {
                 return FuzzyMatcher.class;
         }
+
+        public Class<? extends IdeCrossrefProposalProvider> bindIdeCrossrefProposalProvider() {
+                return ProblemCrossrefProposalProvider.class;
+        }
 }
diff --git a/language-ide/src/main/java/tools/refinery/language/ide/contentassist/ProblemCrossrefProposalProvider.java b/language-ide/src/main/java/tools/refinery/language/ide/contentassist/ProblemCrossrefProposalProvider.java
new file mode 100644
index 00000000..f828e836
--- /dev/null
+++ b/language-ide/src/main/java/tools/refinery/language/ide/contentassist/ProblemCrossrefProposalProvider.java
@@ -0,0 +1,77 @@
+package tools.refinery.language.ide.contentassist;
+
+import java.util.Objects;
+
+import org.eclipse.emf.ecore.EObject;
+import org.eclipse.emf.ecore.util.EcoreUtil;
+import org.eclipse.xtext.CrossReference;
+import org.eclipse.xtext.GrammarUtil;
+import org.eclipse.xtext.ide.editor.contentassist.ContentAssistContext;
+import org.eclipse.xtext.ide.editor.contentassist.ContentAssistEntry;
+import org.eclipse.xtext.ide.editor.contentassist.IdeCrossrefProposalProvider;
+import org.eclipse.xtext.nodemodel.util.NodeModelUtils;
+import org.eclipse.xtext.resource.IEObjectDescription;
+
+import com.google.inject.Inject;
+
+import tools.refinery.language.model.ProblemUtil;
+import tools.refinery.language.model.problem.Problem;
+import tools.refinery.language.resource.ReferenceCounter;
+
+public class ProblemCrossrefProposalProvider extends IdeCrossrefProposalProvider {
+        @Inject
+        private ReferenceCounter referenceCounter;
+
+        @Override
+        protected ContentAssistEntry createProposal(IEObjectDescription candidate, CrossReference crossRef,
+                        ContentAssistContext context) {
+                if (!shouldCreateProposal(candidate, crossRef, context)) {
+                        return null;
+                }
+                return super.createProposal(candidate, crossRef, context);
+        }
+
+        protected boolean shouldCreateProposal(IEObjectDescription candidate, CrossReference crossRef,
+                        ContentAssistContext context) {
+                var rootModel = context.getRootModel();
+                var eObjectOrProxy = candidate.getEObjectOrProxy();
+                if (!Objects.equals(rootModel.eResource(), eObjectOrProxy.eResource())) {
+                        return true;
+                }
+                var currentValue = getCurrentValue(crossRef, context);
+                if (currentValue == null) {
+                        return true;
+                }
+                var eObject = EcoreUtil.resolve(eObjectOrProxy, rootModel);
+                if (!Objects.equals(currentValue, eObject)) {
+                        return true;
+                }
+                if (!ProblemUtil.isImplicit(eObject)) {
+                        return true;
+                }
+                if (rootModel instanceof Problem problem) {
+                        return referenceCounter.countReferences(problem, eObject) >= 2;
+                }
+                return true;
+        }
+
+        protected EObject getCurrentValue(CrossReference crossRef, ContentAssistContext context) {
+                var value = getCurrentValue(crossRef, context.getCurrentModel());
+                if (value != null) {
+                        return value;
+                }
+                var currentNodeSemanticObject = NodeModelUtils.findActualSemanticObjectFor(context.getCurrentNode());
+                return getCurrentValue(crossRef, currentNodeSemanticObject);
+        }
+
+        protected EObject getCurrentValue(CrossReference crossRef, EObject context) {
+                if (context == null) {
+                        return null;
+                }
+                var eReference = GrammarUtil.getReference(crossRef, context.eClass());
+                if (eReference == null || eReference.isMany()) {
+                        return null;
+                }
+                return (EObject) context.eGet(eReference);
+        }
+}
diff --git a/language-model/src/main/java/tools/refinery/language/model/ProblemUtil.java b/language-model/src/main/java/tools/refinery/language/model/ProblemUtil.java
index b6b199f8..5f8641bf 100644
--- a/language-model/src/main/java/tools/refinery/language/model/ProblemUtil.java
+++ b/language-model/src/main/java/tools/refinery/language/model/ProblemUtil.java
@@ -12,6 +12,7 @@ import org.eclipse.emf.ecore.EObject;
 import org.eclipse.emf.ecore.resource.Resource;
 
 import tools.refinery.language.model.problem.ClassDeclaration;
+import tools.refinery.language.model.problem.ImplicitVariable;
 import tools.refinery.language.model.problem.Node;
 import tools.refinery.language.model.problem.Problem;
 import tools.refinery.language.model.problem.ProblemPackage;
@@ -33,6 +34,24 @@ public final class ProblemUtil {
         public static boolean isSingletonVariable(Variable variable) {
                 return variable.eContainingFeature() == ProblemPackage.Literals.VARIABLE_OR_NODE_ARGUMENT__SINGLETON_VARIABLE;
         }
+        
+        public static boolean isImplicitVariable(Variable variable) {
+                return variable instanceof ImplicitVariable;
+        }
+        
+        public static boolean isImplicitNode(Node node) {
+                return node.eContainingFeature() == ProblemPackage.Literals.PROBLEM__NODES;
+        }
+        
+        public static boolean isImplicit(EObject eObject) {
+                if (eObject instanceof Node node) {
+                        return isImplicitNode(node);
+                } else if (eObject instanceof Variable variable) {
+                        return isImplicitVariable(variable);
+                } else {
+                        return false;
+                }
+        }
 
         public static boolean isUniqueNode(Node node) {
                 var containingFeature = node.eContainingFeature();
diff --git a/language-web/package.json b/language-web/package.json
index cb860c5f..3362a47a 100644
--- a/language-web/package.json
+++ b/language-web/package.json
@@ -85,6 +85,7 @@
     "escape-string-regexp": "^5.0.0",
     "@fontsource/jetbrains-mono": "^4.5.0",
     "@fontsource/roboto": "^4.5.1",
+    "@lezer/common": "^0.15.7",
     "@lezer/lr": "^0.15.4",
     "loglevel": "^1.7.1",
     "loglevel-plugin-prefix": "^0.8.4",
diff --git a/language-web/src/main/js/editor/EditorParent.ts b/language-web/src/main/js/editor/EditorParent.ts
index ee1323f6..2d74b863 100644
--- a/language-web/src/main/js/editor/EditorParent.ts
+++ b/language-web/src/main/js/editor/EditorParent.ts
@@ -34,6 +34,9 @@ export const EditorParent = styled('div')(({ theme }) => {
     '&, .cm-editor': {
       height: '100%',
     },
+    '.cm-content': {
+      padding: 0,
+    },
     '.cm-scroller, .cm-tooltip-autocomplete, .cm-completionLabel, .cm-completionDetail': {
       fontSize: 16,
       fontFamily: '"JetBrains MonoVariable", "JetBrains Mono", monospace',
@@ -46,7 +49,7 @@ export const EditorParent = styled('div')(({ theme }) => {
       color: theme.palette.text.secondary,
     },
     '.cm-gutters': {
-      background: theme.palette.background.default,
+      background: 'rgba(255, 255, 255, 0.1)',
       color: theme.palette.text.disabled,
       border: 'none',
     },
@@ -57,7 +60,9 @@ export const EditorParent = styled('div')(({ theme }) => {
       background: 'rgba(0, 0, 0, 0.3)',
     },
     '.cm-activeLineGutter': {
-      background: 'rgba(0, 0, 0, 0.3)',
+      background: 'transparent',
+    },
+    '.cm-lineNumbers .cm-activeLineGutter': {
       color: theme.palette.text.primary,
     },
     '.cm-cursor, .cm-cursor-primary': {
diff --git a/language-web/src/main/js/editor/EditorStore.ts b/language-web/src/main/js/editor/EditorStore.ts
index ba31efcb..059233f4 100644
--- a/language-web/src/main/js/editor/EditorStore.ts
+++ b/language-web/src/main/js/editor/EditorStore.ts
@@ -112,8 +112,8 @@ export class EditorStore {
         }),
         semanticHighlighting,
         // We add the gutters to `extensions` in the order we want them to appear.
-        foldGutter(),
         lineNumbers(),
+        foldGutter(),
         keymap.of([
           ...closeBracketsKeymap,
           ...commentKeymap,
diff --git a/language-web/src/main/js/language/problem.grammar b/language-web/src/main/js/language/problem.grammar
index c242a4ba..8e39243f 100644
--- a/language-web/src/main/js/language/problem.grammar
+++ b/language-web/src/main/js/language/problem.grammar
@@ -1,3 +1,7 @@
+@detectDelim
+
+@external prop implicitCompletion from '../../../../src/main/js/language/props.ts'
+
 @top Problem { statement* }
 
 statement {
@@ -24,7 +28,7 @@ statement {
     RuleBody { ":" sep<OrOp, Conjunction> "~>" sep<OrOp, Action> "." }
   } |
   Assertion {
-    ckw<"default">? (NotOp | UnknownOp)? RelationName
+    kw<"default">? (NotOp | UnknownOp)? RelationName
     ParameterList<AssertionArgument> (":" LogicValue)? "."
   } |
   NodeValueAssertion {
@@ -34,7 +38,7 @@ statement {
     ckw<"unique"> sep<",", UniqueNodeName> "."
   } |
   ScopeDeclaration {
-    ckw<"scope"> sep<",", ScopeElement> "."
+    kw<"scope"> sep<",", ScopeElement> "."
   }
 }
 
@@ -89,11 +93,11 @@ VariableName { QualifiedName }
 
 NodeName { QualifiedName }
 
-QualifiedName { identifier ("::" identifier)* }
+QualifiedName[implicitCompletion=true] { identifier ("::" identifier)* }
 
-kw<term> { @specialize[@name={term}]<identifier, term> }
+kw<term> { @specialize[@name={term},implicitCompletion=true]<identifier, term> }
 
-ckw<term> { @extend[@name={term}]<identifier, term> }
+ckw<term> { @extend[@name={term},implicitCompletion=true]<identifier, term> }
 
 ParameterList<content> { "(" sep<",", content> ")" }
 
diff --git a/language-web/src/main/js/language/props.ts b/language-web/src/main/js/language/props.ts
new file mode 100644
index 00000000..8e488bf5
--- /dev/null
+++ b/language-web/src/main/js/language/props.ts
@@ -0,0 +1,7 @@
+import { NodeProp } from '@lezer/common';
+
+export const implicitCompletion = new NodeProp({
+  deserialize(s: string) {
+    return s === 'true';
+  },
+});
diff --git a/language-web/src/main/js/xtext/ContentAssistService.ts b/language-web/src/main/js/xtext/ContentAssistService.ts
index f085c5b1..aa9a80b0 100644
--- a/language-web/src/main/js/xtext/ContentAssistService.ts
+++ b/language-web/src/main/js/xtext/ContentAssistService.ts
@@ -3,9 +3,11 @@ import type {
   CompletionContext,
   CompletionResult,
 } from '@codemirror/autocomplete';
+import { syntaxTree } from '@codemirror/language';
 import type { Transaction } from '@codemirror/state';
 import escapeStringRegexp from 'escape-string-regexp';
 
+import { implicitCompletion } from '../language/props';
 import type { UpdateService } from './UpdateService';
 import { getLogger } from '../utils/logger';
 import type { IContentAssistEntry } from './xtextServiceResults';
@@ -14,12 +16,57 @@ const PROPOSALS_LIMIT = 1000;
 
 const IDENTIFIER_REGEXP_STR = '[a-zA-Z0-9_]*';
 
-const HIGH_PRIORITY_KEYWORDS = ['<->'];
-
-const QUALIFIED_NAME_SEPARATOR_REGEXP = /::/g;
+const HIGH_PRIORITY_KEYWORDS = ['<->', '~>'];
 
 const log = getLogger('xtext.ContentAssistService');
 
+interface IFoundToken {
+  from: number;
+
+  to: number;
+
+  implicitCompletion: boolean;
+
+  text: string;
+}
+
+function findToken({ pos, state }: CompletionContext): IFoundToken | null {
+  const token = syntaxTree(state).resolveInner(pos, -1);
+  if (token === null) {
+    return null;
+  }
+  if (token.firstChild !== null) {
+    // We only autocomplete terminal nodes. If the current node is nonterminal,
+    // returning `null` makes us autocomplete with the empty prefix instead.
+    return null;
+  }
+  return {
+    from: token.from,
+    to: token.to,
+    implicitCompletion: token.type.prop(implicitCompletion) || false,
+    text: state.sliceDoc(token.from, token.to),
+  };
+}
+
+function shouldCompleteImplicitly(token: IFoundToken | null, context: CompletionContext): boolean {
+  return token !== null
+    && token.implicitCompletion
+    && context.pos - token.from >= 2;
+}
+
+function computeSpan(prefix: string, entryCount: number): RegExp {
+  const escapedPrefix = escapeStringRegexp(prefix);
+  if (entryCount < PROPOSALS_LIMIT) {
+    // Proposals with the current prefix fit the proposals limit.
+    // We can filter client side as long as the current prefix is preserved.
+    return new RegExp(`^${escapedPrefix}${IDENTIFIER_REGEXP_STR}$`);
+  }
+  // The current prefix overflows the proposals limits,
+  // so we have to fetch the completions again on the next keypress.
+  // Hopefully, it'll return a shorter list and we'll be able to filter client side.
+  return new RegExp(`^${escapedPrefix}$`);
+}
+
 function createCompletion(entry: IContentAssistEntry): Completion {
   let boost;
   switch (entry.kind) {
@@ -33,7 +80,7 @@ function createCompletion(entry: IContentAssistEntry): Completion {
       break;
     default: {
       // Penalize qualified names (vs available unqualified names).
-      const extraSegments = entry.proposal.match(QUALIFIED_NAME_SEPARATOR_REGEXP)?.length || 0;
+      const extraSegments = entry.proposal.match(/::/g)?.length || 0;
       boost = Math.max(-5 * extraSegments, -50);
     }
       break;
@@ -47,19 +94,6 @@ function createCompletion(entry: IContentAssistEntry): Completion {
   };
 }
 
-function computeSpan(prefix: string, entryCount: number) {
-  const escapedPrefix = escapeStringRegexp(prefix);
-  if (entryCount < PROPOSALS_LIMIT) {
-    // Proposals with the current prefix fit the proposals limit.
-    // We can filter client side as long as the current prefix is preserved.
-    return new RegExp(`^${escapedPrefix}${IDENTIFIER_REGEXP_STR}$`);
-  }
-  // The current prefix overflows the proposals limits,
-  // so we have to fetch the completions again on the next keypress.
-  // Hopefully, it'll return a shorter list and we'll be able to filter client side.
-  return new RegExp(`^${escapedPrefix}$`);
-}
-
 export class ContentAssistService {
   private readonly updateService: UpdateService;
 
@@ -76,16 +110,16 @@ export class ContentAssistService {
   }
 
   async contentAssist(context: CompletionContext): Promise<CompletionResult> {
-    const tokenBefore = context.tokenBefore(['QualifiedName']);
+    const tokenBefore = findToken(context);
+    if (!context.explicit && !shouldCompleteImplicitly(tokenBefore, context)) {
+      return {
+        from: context.pos,
+        options: [],
+      };
+    }
     let range: { from: number, to: number };
     let prefix = '';
     if (tokenBefore === null) {
-      if (!context.explicit) {
-        return {
-          from: context.pos,
-          options: [],
-        };
-      }
       range = {
         from: context.pos,
         to: context.pos,
@@ -124,7 +158,12 @@ export class ContentAssistService {
     }
     const options: Completion[] = [];
     entries.forEach((entry) => {
-      options.push(createCompletion(entry));
+      if (prefix === entry.prefix) {
+        // Xtext will generate completions that do not complete the current token,
+        // e.g., `(` after trying to complete an indetifier,
+        // but we ignore those, since CodeMirror won't filter for them anyways.
+        options.push(createCompletion(entry));
+      }
     });
     log.debug('Fetched', options.length, 'completions from server');
     this.lastCompletion = {
@@ -137,7 +176,7 @@ export class ContentAssistService {
 
   private shouldReturnCachedCompletion(
     token: { from: number, to: number, text: string } | null,
-  ) {
+  ): boolean {
     if (token === null || this.lastCompletion === null) {
       return false;
     }
@@ -147,10 +186,13 @@ export class ContentAssistService {
       return true;
     }
     const [transformedFrom, transformedTo] = this.mapRangeInclusive(lastFrom, lastTo);
-    return from >= transformedFrom && to <= transformedTo && span && span.exec(text);
+    return from >= transformedFrom
+      && to <= transformedTo
+      && typeof span !== 'undefined'
+      && span.exec(text) !== null;
   }
 
-  private shouldInvalidateCachedCompletion(transaction: Transaction) {
+  private shouldInvalidateCachedCompletion(transaction: Transaction): boolean {
     if (!transaction.docChanged || this.lastCompletion === null) {
       return false;
     }
diff --git a/language-web/yarn.lock b/language-web/yarn.lock
index e05f4fa6..06b30508 100644
--- a/language-web/yarn.lock
+++ b/language-web/yarn.lock
@@ -1476,7 +1476,7 @@
   resolved "https://registry.yarnpkg.com/@humanwhocodes/object-schema/-/object-schema-1.2.0.tgz#87de7af9c231826fdd68ac7258f77c429e0e5fcf"
   integrity sha512-wdppn25U8z/2yiaT6YGquE6X8sSv7hNMWSXYSSU1jGv/yd6XqjXgTDJ8KP4NgjTXfJ3GbRjeeb8RTV7a/VpM+w==
 
-"@lezer/common@^0.15.0", "@lezer/common@^0.15.5":
+"@lezer/common@^0.15.0", "@lezer/common@^0.15.5", "@lezer/common@^0.15.7":
   version "0.15.7"
   resolved "https://registry.yarnpkg.com/@lezer/common/-/common-0.15.7.tgz#8b445dae9777f689783132cf490770ece3c03d7b"
   integrity sha512-Rw8TDJnBzZnkyzIXs1Tmmd241FrBLJBj8gkdy3y0joGFb8Z4I/joKEsR+gv1pb13o1TMsZxm3fmP+d/wPt2CTQ==
diff --git a/language/src/main/java/tools/refinery/language/Problem.xtext b/language/src/main/java/tools/refinery/language/Problem.xtext
index 6f6a8588..0fa47d63 100644
--- a/language/src/main/java/tools/refinery/language/Problem.xtext
+++ b/language/src/main/java/tools/refinery/language/Problem.xtext
@@ -154,7 +154,7 @@ ScopeDeclaration:
         "scope" typeScopes+=TypeScope ("," typeScopes+=TypeScope)* ".";
 
 TypeScope:
-        targetType=[ClassDeclaration]
+        targetType=[ClassDeclaration|QualifiedName]
         (increment?="+=" | "=")
         multiplicity=DefiniteMultiplicity;
 
@@ -183,8 +183,8 @@ QualifiedName hidden():
         Identifier ("::" Identifier)*;
 
 Identifier:
-        ID | "true" | "false" | "unknown" | "error" | "class" | "abstract" | "extends" | "enum" | "pred" | "scope" |
-        "unique" | "default" | "problem" | "new" | "delete";
+        ID | "true" | "false" | "unknown" | "error" | "class" | "abstract" | "extends" | "enum" | "pred" |
+        "unique" | "problem" | "new" | "delete";
 
 Integer returns ecore::EInt hidden():
         "-"? INT;
diff --git a/language/src/main/java/tools/refinery/language/resource/ReferenceCounter.java b/language/src/main/java/tools/refinery/language/resource/ReferenceCounter.java
new file mode 100644
index 00000000..7525dfc6
--- /dev/null
+++ b/language/src/main/java/tools/refinery/language/resource/ReferenceCounter.java
@@ -0,0 +1,51 @@
+package tools.refinery.language.resource;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.eclipse.emf.ecore.EObject;
+import org.eclipse.xtext.util.IResourceScopeCache;
+
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+
+import tools.refinery.language.model.problem.Problem;
+
+@Singleton
+public class ReferenceCounter {
+        @Inject
+        private IResourceScopeCache cache;
+
+        public int countReferences(Problem problem, EObject eObject) {
+                var count = getReferenceCounts(problem).get(eObject);
+                if (count == null) {
+                        return 0;
+                }
+                return count;
+        }
+
+        protected Map<EObject, Integer> getReferenceCounts(Problem problem) {
+                var resource = problem.eResource();
+                if (resource == null) {
+                        return doGetReferenceCounts(problem);
+                }
+                return cache.get(problem, resource, () -> doGetReferenceCounts(problem));
+        }
+
+        protected Map<EObject, Integer> doGetReferenceCounts(Problem problem) {
+                var map = new HashMap<EObject, Integer>();
+                countCrossReferences(problem, map);
+                var iterator = problem.eAllContents();
+                while (iterator.hasNext()) {
+                        var eObject = iterator.next();
+                        countCrossReferences(eObject, map);
+                }
+                return map;
+        }
+
+        protected void countCrossReferences(EObject eObject, Map<EObject, Integer> map) {
+                for (var referencedObject : eObject.eCrossReferences()) {
+                        map.compute(referencedObject, (key, currentValue) -> currentValue == null ? 1 : currentValue + 1);
+                }
+        }
+}
diff --git a/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java b/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java
index f2378df6..dfd386f5 100644
--- a/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java
+++ b/language/src/main/java/tools/refinery/language/validation/ProblemValidator.java
@@ -3,6 +3,19 @@
  */
 package tools.refinery.language.validation;
 
+import org.eclipse.xtext.EcoreUtil2;
+import org.eclipse.xtext.validation.Check;
+
+import com.google.inject.Inject;
+
+import tools.refinery.language.model.ProblemUtil;
+import tools.refinery.language.model.problem.Node;
+import tools.refinery.language.model.problem.Problem;
+import tools.refinery.language.model.problem.ProblemPackage;
+import tools.refinery.language.model.problem.Variable;
+import tools.refinery.language.model.problem.VariableOrNodeArgument;
+import tools.refinery.language.resource.ReferenceCounter;
+
 /**
  * This class contains custom validation rules.
  *
@@ -10,4 +23,40 @@ package tools.refinery.language.validation;
  * https://www.eclipse.org/Xtext/documentation/303_runtime_concepts.html#validation
  */
 public class ProblemValidator extends AbstractProblemValidator {
+        private static final String ISSUE_PREFIX = "tools.refinery.language.validation.ProblemValidator.";
+
+        public static final String SINGLETON_VARIABLE_ISSUE = ISSUE_PREFIX + "SINGLETON_VARIABLE";
+
+        public static final String NON_UNIQUE_NODE_ISSUE = ISSUE_PREFIX + "NON_UNIQUE_NODE";
+
+        @Inject
+        private ReferenceCounter referenceCounter;
+
+        @Check
+        public void checkUniqueVariable(VariableOrNodeArgument argument) {
+                var variableOrNode = argument.getVariableOrNode();
+                if (variableOrNode instanceof Variable variable && ProblemUtil.isImplicitVariable(variable)
+                                && !ProblemUtil.isSingletonVariable(variable)) {
+                        var problem = EcoreUtil2.getContainerOfType(variable, Problem.class);
+                        if (problem != null && referenceCounter.countReferences(problem, variable) <= 1) {
+                                var name = variable.getName();
+                                var message = "Variable '%s' has only a single reference. Add another reference or mark is as a singleton variable: '_%s'"
+                                                .formatted(name, name);
+                                warning(message, argument, ProblemPackage.Literals.VARIABLE_OR_NODE_ARGUMENT__VARIABLE_OR_NODE,
+                                                INSIGNIFICANT_INDEX, SINGLETON_VARIABLE_ISSUE);
+                        }
+                }
+        }
+
+        @Check
+        public void checkNonUniqueNode(VariableOrNodeArgument argument) {
+                var variableOrNode = argument.getVariableOrNode();
+                if (variableOrNode instanceof Node node && !ProblemUtil.isUniqueNode(node)) {
+                        var name = node.getName();
+                        var message = "Only unique nodes can be referenced in predicates. Mark '%s' as unique with the declaration 'unique %s.'"
+                                        .formatted(name, name);
+                        error(message, argument, ProblemPackage.Literals.VARIABLE_OR_NODE_ARGUMENT__VARIABLE_OR_NODE,
+                                        INSIGNIFICANT_INDEX, NON_UNIQUE_NODE_ISSUE);
+                }
+        }
 }