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diff --git a/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/index/IdentityIndexer.java b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/index/IdentityIndexer.java
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+++ b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/index/IdentityIndexer.java
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+/*******************************************************************************
+ * Copyright (c) 2004-2012 Gabor Bergmann and Daniel Varro
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-v20.html.
+ * 
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+
+package tools.refinery.viatra.runtime.rete.index;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.List;
+
+import tools.refinery.viatra.runtime.matchers.tuple.Tuple;
+import tools.refinery.viatra.runtime.matchers.tuple.TupleMask;
+import tools.refinery.viatra.runtime.matchers.util.Direction;
+import tools.refinery.viatra.runtime.rete.network.Node;
+import tools.refinery.viatra.runtime.rete.network.ReteContainer;
+import tools.refinery.viatra.runtime.rete.network.Supplier;
+import tools.refinery.viatra.runtime.rete.network.communication.Timestamp;
+
+/**
+ * Defines an abstract trivial indexer that identically projects the contents of some stateful node, and can therefore
+ * save space. Can only exist in connection with a stateful store, and must be operated by another node (the active
+ * node). Do not attach parents directly!
+ * 
+ * @author Gabor Bergmann
+ * @noimplement Rely on the provided implementations
+ * @noreference Use only via standard Node and Indexer interfaces
+ * @noinstantiate This class is not intended to be instantiated by clients.
+ */
+public abstract class IdentityIndexer extends SpecializedProjectionIndexer {
+
+    protected abstract Collection<Tuple> getTuples();
+
+    public IdentityIndexer(ReteContainer reteContainer, int tupleWidth, Supplier parent, 
+            Node activeNode, List<ListenerSubscription> sharedSubscriptionList) {
+        super(reteContainer, TupleMask.identity(tupleWidth), parent, activeNode, sharedSubscriptionList);
+    }
+
+    @Override
+    public Collection<Tuple> get(Tuple signature) {
+        if (contains(signature)) {
+            return Collections.singleton(signature);
+        } else
+            return null;
+    }
+    
+    protected boolean contains(Tuple signature) {
+        return getTuples().contains(signature);
+    }
+
+    @Override
+    public Collection<Tuple> getSignatures() {
+        return getTuples();
+    }
+    
+    @Override
+    public int getBucketCount() {
+        return getTuples().size();
+    }
+    
+    @Override
+    public Iterator<Tuple> iterator() {
+        return getTuples().iterator();
+    }
+    
+    @Override
+    public void propagateToListener(IndexerListener listener, Direction direction, Tuple updateElement, Timestamp timestamp) {
+        listener.notifyIndexerUpdate(direction, updateElement, updateElement, true, timestamp);
+    }
+
+}
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