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diff --git a/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/itc/igraph/IGraphDataSource.java b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/itc/igraph/IGraphDataSource.java
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+++ b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/itc/igraph/IGraphDataSource.java
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+/*******************************************************************************
+ * Copyright (c) 2010-2012, Tamas Szabo, Istvan Rath 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.itc.igraph;
+
+import tools.refinery.viatra.runtime.matchers.util.IMemoryView;
+import tools.refinery.viatra.runtime.matchers.util.IMultiset;
+
+import java.util.Set;
+
+/**
+ * The interface prescribes the set of operations that a graph data source must support.
+ * <p> Note that the old version of the interface is broken at version 1.6;
+ *  MultiSets are now presented as Maps instead of Lists.
+ *
+ * @author Tamas Szabo
+ *
+ * @param <V>
+ *            the type of the nodes in the graph
+ */
+public interface IGraphDataSource<V> {
+
+    /**
+     * Attaches a new graph observer to this graph data source. Observers will be notified in the order they have been registered.
+     *
+     * @param observer the graph observer
+     */
+    public void attachObserver(IGraphObserver<V> observer);
+
+    /**
+     * Attaches a new graph observer to this graph data source as the first one.
+     * In the notification order this observer will be the first one as long as another call to this method happens.
+     *
+     * @param observer the graph observer
+     * @since 1.6
+     */
+    public void attachAsFirstObserver(IGraphObserver<V> observer);
+
+    /**
+     * Detaches an already registered graph observer from this graph data source.
+     *
+     * @param observer the graph observer
+     */
+    public void detachObserver(IGraphObserver<V> observer);
+
+    /**
+     * Returns the complete set of nodes in the graph data source.
+     *
+     * @return the set of all nodes
+     */
+    public Set<V> getAllNodes();
+
+    /**
+     * Returns the target nodes for the given source node.
+     * The returned data structure is an {@link IMultiset} because of potential parallel edges in the graph data source.
+     *
+     * The method must not return null.
+     *
+     * @param source the source node
+     * @return the multiset of target nodes
+     * @since 2.0
+     */
+    public IMemoryView<V> getTargetNodes(V source);
+}