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diff --git a/bundles/org.simantics.utils.datastructures/src/org/simantics/utils/datastructures/persistent/ImmutableSet.java b/bundles/org.simantics.utils.datastructures/src/org/simantics/utils/datastructures/persistent/ImmutableSet.java
new file mode 100644
index 000000000..3ee830b3d
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+++ b/bundles/org.simantics.utils.datastructures/src/org/simantics/utils/datastructures/persistent/ImmutableSet.java
@@ -0,0 +1,445 @@
+/*******************************************************************************
+ * 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.persistent;
+
+import gnu.trove.procedure.TObjectProcedure;
+
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Random;
+import java.util.Set;
+
+public class ImmutableSet<T extends Comparable<T>> {
+	
+	@SuppressWarnings({ "rawtypes" })
+	static final ImmutableSet EMPTY = new EmptyImmutableSet();
+	
+	private static class EmptyImmutableSet<T extends Comparable<T>> extends ImmutableSet<T> {
+		public EmptyImmutableSet() {
+			isBlack = true;
+		}
+		
+		protected org.simantics.utils.datastructures.persistent.ImmutableSet<T> addRec(T obj) {
+			return new ImmutableSet<T>(obj);
+		}
+		
+		protected org.simantics.utils.datastructures.persistent.ImmutableSet<T> removeRec(T obj) {
+			return null;
+		}
+		
+		public boolean contains(T obj) {
+			return false;
+		}
+		
+		@Override
+		public boolean forEach(TObjectProcedure<T> arg0) {
+			return true;
+		}
+		
+		@Override
+		void print(int arg0) {		
+		}
+	}
+	
+	ImmutableSet<T> left;
+	T key;
+	ImmutableSet<T> right;
+	boolean isBlack;
+	
+	protected ImmutableSet(
+			ImmutableSet<T> left,
+			T key,
+			ImmutableSet<T> right,			
+			boolean isBlack) {
+		this.left = left;
+		this.right = right;
+		this.key = key;
+		this.isBlack = isBlack;
+	}
+
+	@SuppressWarnings("unchecked")
+	public ImmutableSet(T key) {
+		this(EMPTY, key, EMPTY, false);
+	}
+	
+	@SuppressWarnings("unchecked")
+	public static <T extends Comparable<T>> ImmutableSet<T> of(T ... array) {
+		if(array.length == 0)
+			return EMPTY;
+		ImmutableSet<T> ret = new ImmutableSet<T>(array[0]);
+		for(int i=1;i<array.length;++i)
+			ret = ret.add(array[i]);
+		return ret;
+	}
+	
+	@SuppressWarnings("unchecked")
+	public static <T extends Comparable<T>> ImmutableSet<T> of(Collection<T> c) {
+		Iterator<T> it = c.iterator();
+		if(!it.hasNext())
+			return EMPTY;		
+		ImmutableSet<T> ret = new ImmutableSet<T>(it.next());
+		while(it.hasNext())
+			ret = ret.add(it.next());
+		return ret;
+	}
+
+	private ImmutableSet() {	
+	}
+	
+	public boolean contains(T obj) {
+		int cmp = obj.compareTo(key);
+		if(cmp < 0)
+			return left.contains(obj);
+		else if(cmp > 0)
+			return right.contains(obj);
+		else
+			return true;
+	}	
+	
+	protected ImmutableSet<T> addRec(T obj) {
+		int cmp = obj.compareTo(key);
+		if(cmp < 0) {
+			ImmutableSet<T> newLeft = left.addRec(obj);
+			if(newLeft == left)
+				return this;			
+			if(isBlack)
+				return balance(newLeft, key, right);
+			else
+				return red(newLeft, key, right);
+		}
+		else if(cmp > 0) {
+			ImmutableSet<T> newRight = right.addRec(obj);
+			if(newRight == right)
+				return this;
+			if(isBlack)
+				return balance(left, key, newRight);
+			else
+				return red(left, key, newRight);
+		}
+		else
+			return this;
+	}
+	
+	/**
+	 * Assumes this is a black nonempty node. 
+	 * 
+	 * Removes obj from the tree. The returned tree has always
+	 * one black node less in every branch (even if nothing is
+	 * removed).
+	 *   
+	 * @param obj
+	 * @return
+	 */
+	protected ImmutableSet<T> removeRec(T obj) {		
+		int cmp = obj.compareTo(key);
+		if(cmp < 0) {
+			ImmutableSet<T> newLeft = left.removeRec(obj);
+			if(newLeft == null)
+				return null;
+			else if(left.isBlack)
+				return balleft(newLeft, key, right);
+			else
+				return red(newLeft, key, right);
+		}
+		else if(cmp > 0) {
+			ImmutableSet<T> newRight = right.removeRec(obj);
+			if(newRight == null)
+				return null;
+			else if(right.isBlack)
+				return balright(left, key, newRight);
+			else
+				return red(left, key, newRight);			
+		}
+		else
+			return append(left, right);
+	}
+	
+	/**
+	 * Assumes a and b have the same black depth and keys in a are strictly less
+	 * than keys in b. Creates a new tree with the same black depth as a and b. 
+	 * 
+	 * @param <T>
+	 * @param a
+	 * @param b
+	 * @return
+	 */
+	protected static <T extends Comparable<T>> ImmutableSet<T> append(ImmutableSet<T> a, ImmutableSet<T> b) {
+		if(a==EMPTY)
+			return b;
+		if(b==EMPTY)
+			return a;
+		if(a.isBlack) {
+			if(b.isBlack) {
+				ImmutableSet<T> middle = append(a.right, b.left);
+				if(middle.isBlack)
+					return balleft(a.left, a.key, black(middle, b.key, b.right));
+				else
+					return red(black(a.left, a.key, middle.left), middle.key, black(middle.right, b.key, b.right));
+			}
+			else
+				return red(append(a, b.left), b.key, b.right);
+		}
+		else {
+			if(b.isBlack)
+				return red(a.left, a.key, append(a.right, b));
+			else {
+				ImmutableSet<T> middle = append(a.right, b.left);
+				if(middle.isBlack)
+					return red(a.left, a.key, red(middle, b.key, b.right));
+				else
+					return red(red(a.left, a.key, middle.left), middle.key, red(middle.right, b.key, b.right));
+			}
+		}
+	}
+	
+	public T getFirst() {
+		if(left == EMPTY)
+			return key;
+		else
+			return left.getFirst();
+	}	
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> balance(
+			ImmutableSet<T> left,
+			T key,
+			ImmutableSet<T> right) {
+		if(!left.isBlack) {
+			if(!left.left.isBlack) 
+				return red(
+					toBlack(left.left),
+					left.key,
+					black(left.right, key, right)
+				);
+			else if(!left.right.isBlack)
+				return red(
+					black(left.left, left.key, left.right.left),
+					left.right.key,
+					black(left.right.right, key, right)					
+				);				
+		}
+		if(!right.isBlack) {
+			if(!right.left.isBlack)
+				return red(
+					black(left, key, right.left.left),
+					right.left.key,
+					black(right.left.right, right.key, right.right)
+				);
+			else if(!right.right.isBlack)
+				return red(
+					black(left, key, right.left),
+					right.key,
+					toBlack(right.right)
+				);		
+		}
+		return black(left, key, right);
+	}
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> black(
+			ImmutableSet<T> left,
+			T key,
+			ImmutableSet<T> right) {
+		return new ImmutableSet<T>(left, key, right, true);
+	}
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> red(
+			ImmutableSet<T> left,
+			T key,
+			ImmutableSet<T> right) {
+		return new ImmutableSet<T>(left, key, right, false);
+	}
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> toBlack(ImmutableSet<T> tree) {
+		if(tree.isBlack)
+			return tree;
+		else
+			return black(tree.left, tree.key, tree.right);
+	}
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> toRed(ImmutableSet<T> tree) {
+		if(tree.isBlack)
+			return red(tree.left, tree.key, tree.right);
+		else
+			return tree;
+	}
+			
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> balleft(
+			ImmutableSet<T> left,
+			T key,
+			ImmutableSet<T> right) {
+		if(left.isBlack) {
+			if(right.isBlack)
+				return balance(left, key, toRed(right));
+			else
+				return red(black(left, key, right.left.left), right.left.key, balance(right.left.right, right.key, toRed(right.right)));
+		}
+		else
+			return red(toBlack(left), key, right);
+	}
+	
+	static private <T extends Comparable<T>> ImmutableSet<T> balright(
+			ImmutableSet<T> left,
+			T key,
+			ImmutableSet<T> right) {
+		if(right.isBlack) {
+			if(left.isBlack)
+				return balance(toRed(left), key, right);
+			else
+				return red(balance(toRed(left.left), left.key, left.right.left), left.right.key, black(left.right.right, key, right));
+		}
+		else
+			return red(left, key, toBlack(right));
+	}
+
+	public ImmutableSet<T> add(T obj) {
+		ImmutableSet<T> tree = addRec(obj);
+		tree.isBlack = true;
+		return tree;
+	}
+
+	public ImmutableSet<T> remove(T obj) {
+		ImmutableSet<T> tree = removeRec(obj);
+		if(tree == null)
+			return this;
+		if(tree.isBlack)
+			return tree;
+		else
+			return black(tree.left, tree.key, tree.right);
+	}
+
+	public boolean forEach(TObjectProcedure<T> procedure) {
+		if(left.forEach(procedure))
+			if(procedure.execute(key))
+				return right.forEach(procedure);
+		return false;
+	}
+	
+	public ImmutableSet<T> addAll(Iterable<T> c) {
+		ImmutableSet<T> ret = this;
+		for(T t : c)
+			ret = ret.add(t);
+		return ret;
+	}
+	
+	static class AddAll<T extends Comparable<T>> implements TObjectProcedure<T> {
+
+		ImmutableSet<T> result;		
+		
+		public AddAll(ImmutableSet<T> result) {
+			this.result = result;
+		}
+
+		@Override
+		public boolean execute(T arg0) {
+			result = result.add(arg0);
+			return true;
+		}
+		
+	}
+	
+	public ImmutableSet<T> addAll(ImmutableSet<T> c) {
+		if(this==EMPTY)
+			return c;
+		if(c==EMPTY)
+			return this;
+		AddAll<T> proc = new AddAll<T>(this);
+		c.forEach(proc);
+		return proc.result;
+	}
+	
+	/**************************************************************************
+	 * 
+	 *  Testing
+	 * 
+	 ************************************************************************** 
+	 */
+	
+	void print(int indentation) {
+		left.print(indentation + 1);
+		for(int i=0;i<indentation;++i)
+			System.out.print("    ");
+		System.out.println(key + " " + (isBlack ? "BLACK" : "RED"));
+		right.print(indentation + 1);
+	}
+	
+	private int validateRec() {
+		if(this==EMPTY)
+			return 1;
+		int lh = left.validateRec();
+		int rh = right.validateRec();
+		if(lh != rh)
+			System.out.println("Unbalanced!");
+		if(!isBlack) {
+			if(!left.isBlack || !right.isBlack)
+				System.out.println("Red under red");
+			return lh;
+		}
+		else
+			return lh+1;
+	}	
+
+	@SuppressWarnings("unchecked")
+	public static <T extends Comparable<T>> ImmutableSet<T> empty() {
+		return EMPTY;
+	}
+	
+	public void validate() {
+		validateRec();
+		if(!isBlack)
+			System.out.println("Root is not black");
+	}
+
+	@SuppressWarnings("unchecked")
+	public static void main(String[] args) {
+		ImmutableSet<Integer> set = ImmutableSet.EMPTY;
+		final Set<Integer> set2 = new HashSet<Integer>();
+
+		Random random = new Random();
+		for(int i=0;i<10000;++i) {
+			int r1 = random.nextInt(1000);
+			int r2 = random.nextInt(1000);
+			set = set.add(r1);
+			set = set.remove(r2);
+			set2.add(r1);
+			set2.remove(r2);
+			set.validate();			
+			
+			for(Integer v : set2)
+				if(!set.contains(v))
+					System.out.println("set2 has more elements");
+			set.forEach(new TObjectProcedure<Integer>() {
+
+				@Override
+				public boolean execute(Integer arg0) {
+					if(!set2.contains(arg0))
+						System.out.println("set has more elements");
+					return true;
+				}
+				
+			});
+		}
+					
+		/*
+		map.forEach(new TObjectProcedure<Integer>() {
+
+			@Override
+			public boolean execute(Integer arg0) {
+				System.out.println(arg0);
+				return true;
+			}
+			
+		});
+		*/
+	}
+	
+}