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diff --git a/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/itc/igraph/IBiDirectionalGraphDataSource.java b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/itc/igraph/IBiDirectionalGraphDataSource.java
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+++ b/subprojects/viatra-runtime-rete/src/main/java/tools/refinery/viatra/runtime/rete/itc/igraph/IBiDirectionalGraphDataSource.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;
+
+/**
+ * A bi-directional graph data source supports all operations that an {@link IGraphDataSource} does, but it
+ * also makes it possible to query the incoming edges of nodes, not only the outgoing edges.
+ *
+ * @author Tamas Szabo
+ *
+ * @param <V> the type of the nodes in the graph
+ */
+public interface IBiDirectionalGraphDataSource<V> extends IGraphDataSource<V> {
+
+    /**
+     * Returns the source nodes for the given target 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 target the target node
+     * @return the multiset of source nodes
+     * @since 2.0
+     */
+    public IMemoryView<V> getSourceNodes(V target);
+
+}