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diff --git a/bundles/org.simantics.utils.datastructures/src/org/simantics/utils/datastructures/SetUnion.java b/bundles/org.simantics.utils.datastructures/src/org/simantics/utils/datastructures/SetUnion.java
new file mode 100644
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--- /dev/null
+++ b/bundles/org.simantics.utils.datastructures/src/org/simantics/utils/datastructures/SetUnion.java
@@ -0,0 +1,116 @@
+/*******************************************************************************
+ * Copyright (c) 2007, 2010 Association for Decentralized Information Management
+ * in Industry THTH ry.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     VTT Technical Research Centre of Finland - initial API and implementation
+ *******************************************************************************/
+package org.simantics.utils.datastructures;
+
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.Set;
+
+public class SetUnion<T> extends ImmutableCollection<T> {
+
+    private Set<T> set1;
+    private Set<T> set2;    
+    
+    public SetUnion(Set<T> set1, Set<T> set2) {
+        this.set1 = set1;
+        this.set2 = set2;
+    }
+
+    @Override
+    public boolean contains(Object o) {
+        return set1.contains(o) || set2.contains(o);
+    }
+
+    @Override
+    public boolean containsAll(Collection<?> c) {
+        for(Object o : c)
+            if(!contains(o))
+                return false;
+        return true;
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return set1.isEmpty() && set2.isEmpty();
+    }
+
+    @Override
+    public Iterator<T> iterator() {
+        return new ImmutableIterator<T>() {
+
+            int setId = 0;
+            Iterator<T> it1 = set1.iterator();
+            Iterator<T> it2 = set2.iterator();
+            T element = null;
+            
+            @Override
+            public boolean hasNext() {
+                if(setId == 0) {
+                    if(it1.hasNext())
+                        return true;
+                    else {
+                        setId = 1;
+                        return hasNext();
+                    }
+                }
+                else {
+                    if(element != null)
+                        return true;
+                    while(it2.hasNext()) {                            
+                        element = it2.next();
+                        if(set1.contains(element)) 
+                            element = null;
+                        else
+                            return true;
+                    }
+                    return false;
+                }
+            }
+
+            @Override
+            public T next() {
+                if(setId == 0) {
+                    if(it1.hasNext())
+                        return it1.next();
+                    else {
+                        setId = 1;
+                        return next();
+                    }
+                }
+                else {
+                    if(element != null) {
+                        T ret = element;
+                        element = null;
+                        return ret;
+                    }
+                    while(it2.hasNext()) {                            
+                        T ret = it2.next();
+                        if(!set1.contains(ret)) 
+                            return ret;
+                    }
+                    return null;
+                }
+            }
+            
+        };
+    }
+
+    @Override
+    public int size() {
+        int count = set1.size();       
+        for(T o : set2)
+            if(!set1.contains(o))
+                ++count;
+        return count;
+    }       
+    
+}