diff --git a/.cproject b/.cproject
index 6ef68f8f2..55ad7b743 100644
--- a/.cproject
+++ b/.cproject
@@ -317,7 +317,6 @@
 </target>
 <target name="clean" path="wrap" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
-<buildArguments/>
 <buildTarget>clean</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>true</useDefaultCommand>
@@ -477,6 +476,7 @@
 </target>
 <target name="testBayesTree.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
+<buildArguments/>
 <buildTarget>testBayesTree.run</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>false</useDefaultCommand>
@@ -484,7 +484,6 @@
 </target>
 <target name="testSymbolicBayesNet.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
-<buildArguments/>
 <buildTarget>testSymbolicBayesNet.run</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>false</useDefaultCommand>
@@ -492,6 +491,7 @@
 </target>
 <target name="testSymbolicFactorGraph.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
+<buildArguments/>
 <buildTarget>testSymbolicFactorGraph.run</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>false</useDefaultCommand>
@@ -683,6 +683,7 @@
 </target>
 <target name="testGraph.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
+<buildArguments/>
 <buildTarget>testGraph.run</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>false</useDefaultCommand>
@@ -738,7 +739,6 @@
 </target>
 <target name="testSimulated2D.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
-<buildArguments/>
 <buildTarget>testSimulated2D.run</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>false</useDefaultCommand>
@@ -786,11 +786,20 @@
 </target>
 <target name="testErrors.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
+<buildArguments/>
 <buildTarget>testErrors.run</buildTarget>
 <stopOnError>true</stopOnError>
 <useDefaultCommand>false</useDefaultCommand>
 <runAllBuilders>true</runAllBuilders>
 </target>
+<target name="testDSF.run" path="cpp" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
+<buildCommand>make</buildCommand>
+<buildArguments/>
+<buildTarget>testDSF.run</buildTarget>
+<stopOnError>true</stopOnError>
+<useDefaultCommand>true</useDefaultCommand>
+<runAllBuilders>true</runAllBuilders>
+</target>
 <target name="install" path="" targetID="org.eclipse.cdt.build.MakeTargetBuilder">
 <buildCommand>make</buildCommand>
 <buildArguments>-j2</buildArguments>
diff --git a/cpp/BTree.h b/cpp/BTree.h
index 862501bcd..24f45a1af 100644
--- a/cpp/BTree.h
+++ b/cpp/BTree.h
@@ -224,7 +224,8 @@ namespace gtsam {
 				else if (key < k) node = node->right.root_.get();
 				else /* (key() == k) */ return node->value();
 			}
-			throw std::invalid_argument("BTree::find: key '" + (std::string) k + "' not found");
+			//throw std::invalid_argument("BTree::find: key '" + (std::string) k + "' not found");
+			throw std::invalid_argument("BTree::find: key not found");
 		}
 
 		/** print in-order */
diff --git a/cpp/DSF.h b/cpp/DSF.h
new file mode 100644
index 000000000..7d8c2e9f1
--- /dev/null
+++ b/cpp/DSF.h
@@ -0,0 +1,134 @@
+/*
+ * DSF.h
+ *
+ *  Created on: Mar 26, 2010
+ *  Author: nikai
+ *  Description: An implementation of Disjoint set forests (see CLR page 446 and up)
+ *  						 Quoting from CLR: A disjoint-set data structure maintains a collection
+ * 							 S = {S_1,S_2,...} of disjoint dynamic sets. Each set is identified by
+ * 							 a representative, which is some member of the set.
+ */
+
+#pragma once
+
+#include <list>
+#include <set>
+#include <map>
+#include <boost/foreach.hpp>
+#include "BTree.h"
+
+namespace gtsam {
+
+	class Symbol;
+
+	template <class Key>
+	class DSF : BTree<Key, Key> {
+
+	public:
+		typedef Key Label; // label can be different from key, but for now they are same
+		typedef DSF<Key> Self;
+		typedef std::set<Key> Set;
+		typedef BTree<Key, Label> Tree;
+		typedef std::pair<Key, Label> KeyLabel;
+
+		// constructor
+		DSF() : Tree() { }
+
+		// constructor
+		DSF(const Tree& tree) : Tree(tree) {}
+
+		// create a new singleton, does nothing if already exists
+		Self makeSet(const Key& key) const { if (mem(key)) return *this; else return add(key, key); }
+
+		// find the label of the set in which {key} lives
+		Label findSet(const Key& key) const {
+			Key parent = find(key);
+			return parent == key ? key : findSet(parent); }
+
+		// return a new DSF where x and y are in the same set. Kai: the caml implementation is not const, and I followed
+		Self makeUnion(const Key& key1, const Key& key2) { return add(findSet_(key2), findSet_(key1));	}
+
+		// create a new singleton with two connected keys
+		Self makePair(const Key& key1, const Key& key2) const { return makeSet(key1).makeSet(key2).makeUnion(key1, key2); }
+
+		// create a new singleton with a list of fully connected keys
+		Self makeList(const std::list<Key>& keys) const {
+			Self t = *this;
+			BOOST_FOREACH(const Key& key, keys)
+				t = t.makePair(key, keys.front());
+			return t;
+		}
+
+		// return a dsf in which all find_set operations will be O(1) due to path compression.
+		DSF flatten() const {
+			DSF t = *this;
+			BOOST_FOREACH(const KeyLabel& pair, (Tree)t)
+				t.findSet_(pair.first);
+			return t;
+		}
+
+		// maps f over all keys, must be invertible
+		DSF map(boost::function<Key(const Key&)> func) const {
+			DSF t;
+			BOOST_FOREACH(const KeyLabel& pair, (Tree)*this)
+				t.add(func(pair.first), func(pair.second));
+			return t;
+		}
+
+		// return the number of sets
+		size_t numSets() const {
+			size_t num = 0;
+			BOOST_FOREACH(const KeyLabel& pair, (Tree)*this)
+				if (pair.first == pair.second) num++;
+			return num;
+		}
+
+		// return the numer of keys
+		size_t size() const { return Tree::size(); }
+
+		// return all sets, i.e. a partition of all elements
+		std::map<Label, Set> sets() const {
+			std::map<Label, Set> sets;
+			BOOST_FOREACH(const KeyLabel& pair, (Tree)*this)
+				sets[findSet(pair.second)].insert(pair.first);
+			return sets;
+		}
+
+		// return a partition of the given elements {keys}
+		std::map<Label, Set> partition(const std::list<Key>& keys) const {
+			std::map<Label, Set> partitions;
+			BOOST_FOREACH(const Key& key, keys)
+				partitions[findSet(key)].insert(key);
+			return partitions;
+		}
+
+		/** equality */
+		bool operator==(const Self& t) const { return (Tree)*this == (Tree)t;	}
+
+		// print the object
+		void print(std::string& name = "DSF") const { Tree::print(name); }
+
+	private:
+
+		/**
+		 * same as findSet except with path compression: After we have traversed the path to
+		 * the root, each parent pointer is made to directly point to it
+		 */
+		Key findSet_(const Key& key) {
+			Key parent = find(key);
+			if (parent == key)
+				return parent;
+			else {
+				Key label = findSet_(parent);
+				*this = add(key, label);
+				return label;
+			}
+		}
+
+	};
+
+	// shortcuts
+	typedef DSF<int> DSFInt;
+	typedef DSF<Symbol> DSFSymbol;
+
+} // namespace gtsam
diff --git a/cpp/Makefile.am b/cpp/Makefile.am
index d760325c2..82614367f 100644
--- a/cpp/Makefile.am
+++ b/cpp/Makefile.am
@@ -40,13 +40,15 @@ testMatrix_LDADD   = libgtsam.la
 # The header files will be installed in ~/include/gtsam
 headers = gtsam.h Value.h Testable.h Factor.h Conditional.h 
 headers += Ordering.h IndexTable.h numericalDerivative.h
-headers += BTree.h
+headers += BTree.h DSF.h
 sources += Ordering.cpp smallExample.cpp 
-check_PROGRAMS += testOrdering testBTree
+check_PROGRAMS += testOrdering testBTree testDSF
 testOrdering_SOURCES = testOrdering.cpp
 testOrdering_LDADD   = libgtsam.la
 testBTree_SOURCES    = testBTree.cpp
 testBTree_LDADD		   = libgtsam.la
+testDSF_SOURCES    = testDSF.cpp
+testDSF_LDADD      = libgtsam.la
 
 # Symbolic Inference
 headers += SymbolicConditional.h
diff --git a/cpp/testDSF.cpp b/cpp/testDSF.cpp
new file mode 100644
index 000000000..dc5ba300b
--- /dev/null
+++ b/cpp/testDSF.cpp
@@ -0,0 +1,222 @@
+/*
+ * testDSF.cpp
+ *
+ *   Created on: Mar 26, 2010
+ *       Author: nikai
+ *  Description: unit tests for DSF
+ */
+
+#include <iostream>
+#include <boost/assign/std/list.hpp>
+#include <boost/assign/std/set.hpp>
+using namespace boost::assign;
+#include <CppUnitLite/TestHarness.h>
+
+#include "DSF.h"
+
+using namespace std;
+using namespace gtsam;
+
+/* ************************************************************************* */
+TEST(DSF, makeSet) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	LONGS_EQUAL(1, dsf.size());
+}
+
+/* ************************************************************************* */
+TEST(DSF, findSet) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	CHECK(dsf.findSet(5) != dsf.findSet(7));
+}
+
+/* ************************************************************************* */
+TEST(DSF, makeUnion) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,7);
+	CHECK(dsf.findSet(5) == dsf.findSet(7));
+}
+
+/* ************************************************************************* */
+TEST(DSF, makeUnion2) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(7,5);
+	CHECK(dsf.findSet(5) == dsf.findSet(7));
+}
+
+/* ************************************************************************* */
+TEST(DSF, makeUnion3) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+	dsf = dsf.makeUnion(6,7);
+	CHECK(dsf.findSet(5) == dsf.findSet(7));
+}
+
+/* ************************************************************************* */
+TEST(DSF, makePair) {
+	DSFInt dsf;
+	dsf = dsf.makePair(0, 1);
+	dsf = dsf.makePair(1, 2);
+	dsf = dsf.makePair(3, 2);
+	CHECK(dsf.findSet(0) == dsf.findSet(3));
+}
+
+/* ************************************************************************* */
+TEST(DSF, makeList) {
+	DSFInt dsf;
+	list<int> keys; keys += 5, 6, 7;
+	dsf = dsf.makeList(keys);
+	CHECK(dsf.findSet(5) == dsf.findSet(7));
+}
+
+/* ************************************************************************* */
+TEST(DSF, numSets) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+	LONGS_EQUAL(2, dsf.numSets());
+}
+
+/* ************************************************************************* */
+TEST(DSF, sets) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeUnion(5,6);
+	map<int, set<int> > sets = dsf.sets();
+	LONGS_EQUAL(1, sets.size());
+
+	set<int> expected; expected += 5, 6;
+	CHECK(expected == sets[dsf.findSet(5)]);
+}
+
+/* ************************************************************************* */
+TEST(DSF, sets2) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+	dsf = dsf.makeUnion(6,7);
+	map<int, set<int> > sets = dsf.sets();
+	LONGS_EQUAL(1, sets.size());
+
+	set<int> expected; expected += 5, 6, 7;
+	CHECK(expected == sets[dsf.findSet(5)]);
+}
+
+/* ************************************************************************* */
+TEST(DSF, sets3) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+	map<int, set<int> > sets = dsf.sets();
+	LONGS_EQUAL(2, sets.size());
+
+	set<int> expected; expected += 5, 6;
+	CHECK(expected == sets[dsf.findSet(5)]);
+}
+
+/* ************************************************************************* */
+TEST(DSF, partition) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeUnion(5,6);
+
+	list<int> keys; keys += 5;
+	map<int, set<int> > partitions = dsf.partition(keys);
+	LONGS_EQUAL(1, partitions.size());
+
+	set<int> expected; expected += 5;
+	CHECK(expected == partitions[dsf.findSet(5)]);
+}
+
+/* ************************************************************************* */
+TEST(DSF, partition2) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+
+	list<int> keys; keys += 7;
+	map<int, set<int> > partitions = dsf.partition(keys);
+	LONGS_EQUAL(1, partitions.size());
+
+	set<int> expected; expected += 7;
+	CHECK(expected == partitions[dsf.findSet(7)]);
+}
+
+/* ************************************************************************* */
+TEST(DSF, partition3) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+
+	list<int> keys; keys += 5, 7;
+	map<int, set<int> > partitions = dsf.partition(keys);
+	LONGS_EQUAL(2, partitions.size());
+
+	set<int> expected; expected += 5;
+	CHECK(expected == partitions[dsf.findSet(5)]);
+}
+
+/* ************************************************************************* */
+int func(const int& a) { return a + 10; }
+TEST(DSF, map) {
+	DSFInt dsf;
+	dsf = dsf.makeSet(5);
+	dsf = dsf.makeSet(6);
+	dsf = dsf.makeSet(7);
+	dsf = dsf.makeUnion(5,6);
+
+	DSFInt actual = dsf.map(&func);
+	DSFInt expected;
+	dsf = dsf.makeSet(15);
+	dsf = dsf.makeSet(16);
+	dsf = dsf.makeSet(17);
+	dsf = dsf.makeUnion(15,16);
+	CHECK(actual == expected);
+}
+
+/* ************************************************************************* */
+TEST(DSF, flatten) {
+	DSFInt dsf;
+	dsf = dsf.makePair(1, 2);
+	dsf = dsf.makePair(2, 3);
+	dsf = dsf.makePair(5, 6);
+	dsf = dsf.makePair(6, 7);
+	dsf = dsf.makeUnion(2, 6);
+
+	DSFInt actual = dsf.flatten();
+	DSFInt expected;
+	expected = expected.makePair(1, 2);
+	expected = expected.makePair(1, 3);
+	expected = expected.makePair(1, 5);
+	expected = expected.makePair(1, 6);
+	expected = expected.makePair(1, 7);
+	CHECK(actual == expected);
+}
+/* ************************************************************************* */
+int main() { TestResult tr; return TestRegistry::runAllTests(tr);}
+/* ************************************************************************* */
+