diff --git a/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/SetOfStates.java b/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/SetOfStates.java
index 564aeaef3..30b432c63 100644
--- a/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/SetOfStates.java
+++ b/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/SetOfStates.java
@@ -27,32 +27,14 @@ package org.broadinstitute.sting.gatk.walkers.varianteval.stratifications;
 import java.util.List;
 
 /**
-* [Short one sentence description of this walker]
-* <p/>
-* <p>
-* [Functionality of this walker]
-* </p>
-* <p/>
-* <h2>Input</h2>
-* <p>
-* [Input description]
-* </p>
-* <p/>
-* <h2>Output</h2>
-* <p>
-* [Output description]
-* </p>
-* <p/>
-* <h2>Examples</h2>
-* <pre>
-*    java
-*      -jar GenomeAnalysisTK.jar
-*      -T $WalkerName
-*  </pre>
-*
-* @author Your Name
-* @since Date created
-*/
-public interface SetOfStates {
-    public List<String> getAllStates();
+ * A basic interface for a class to be used with the StratificationStates system
+ *
+ * @author Mark DePristo
+ * @since 3/28/12
+ */
+public interface SetOfStates<Object> {
+    /**
+     * @return a list of all objects states that may be provided by this States provider
+     */
+    public List<Object> getAllStates();
 }
diff --git a/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratNode.java b/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratNode.java
index 1a7e2dde7..f350df47d 100644
--- a/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratNode.java
+++ b/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratNode.java
@@ -24,12 +24,12 @@
 
 package org.broadinstitute.sting.gatk.walkers.varianteval.stratifications;
 
+import com.google.java.contract.Ensures;
+import com.google.java.contract.Invariant;
+import com.google.java.contract.Requires;
 import org.broadinstitute.sting.utils.exceptions.ReviewedStingException;
 
-import java.util.Collections;
-import java.util.Iterator;
-import java.util.List;
-import java.util.Map;
+import java.util.*;
 
 /**
  * Helper class representing a tree of stratification splits, where leaf nodes
@@ -49,35 +49,48 @@ import java.util.Map;
  * This code allows us to efficiently look up a state key (A=2, B=3) and map it
  * to a specific key (an integer) that's unique over the tree
  *
+ * Note the structure of this tree is that the keys are -1 for all internal nodes, and
+ * leafs are the only nodes with meaningful keys.  So for a tree with 2N nodes N of these
+ * will be internal, with no keys, and meaningful maps from states -> subtrees.  The
+ * other N nodes are leafs, with meaningful keys, empty maps, and null stratification objects
+ *
  * @author Mark DePristo
  * @since 3/27/12
  */
-public class StratNode<T extends SetOfStates> implements Iterable<StratNode<T>> {
+@Invariant({
+        "(isLeaf() && stratifier == null && subnodes.isEmpty()) || (!isLeaf() && stratifier != null && !subnodes.isEmpty())"})
+class StratNode<T extends SetOfStates> implements Iterable<StratNode<T>> {
     int key = -1;
     final T stratifier;
-    final Map<String, StratNode<T>> subnodes;
+    final Map<Object, StratNode<T>> subnodes;
 
-    public StratNode() {
+    protected StratNode() {
         this.subnodes = Collections.emptyMap();
         this.stratifier = null;
     }
 
-    StratNode(final T stratifier, final Map<String, StratNode<T>> subnodes) {
+    protected StratNode(final T stratifier, final Map<Object, StratNode<T>> subnodes) {
         this.stratifier = stratifier;
         this.subnodes = subnodes;
     }
 
+    @Requires("key >= 0")
     public void setKey(final int key) {
         if ( ! isLeaf() )
             throw new ReviewedStingException("Cannot set key of non-leaf node");
         this.key = key;
     }
 
-    public int find(final List<String> states, int offset) {
+    @Requires({
+            "states != null",
+            "offset >= 0",
+            "offset <= states.size()"
+            })
+    public int find(final List<Object> states, int offset) {
         if ( isLeaf() ) // we're here!
             return key;
         else {
-            final String state = states.get(offset);
+            final Object state = states.get(offset);
             StratNode<T> subnode = subnodes.get(state);
             if ( subnode == null )
                 throw new ReviewedStingException("Couldn't find state for " + state + " at node " + this);
@@ -86,6 +99,28 @@ public class StratNode<T extends SetOfStates> implements Iterable<StratNode<T>>
         }
     }
 
+    @Requires({
+            "multipleStates != null",
+            "offset >= 0",
+            "offset <= multipleStates.size()",
+            "keys != null",
+            "offset == multipleStates.size() || multipleStates.get(offset) != null"})
+    public void find(final List<List<Object>> multipleStates, final int offset, final HashSet<Integer> keys) {
+        if ( isLeaf() ) // we're here!
+            keys.add(key);
+        else {
+            for ( final Object state : multipleStates.get(offset) ) {
+                // loop over all of the states at this offset
+                final StratNode<T> subnode = subnodes.get(state);
+                if ( subnode == null )
+                    throw new ReviewedStingException("Couldn't find state for " + state + " at node " + this);
+                else
+                    subnode.find(multipleStates, offset+1, keys);
+            }
+        }
+    }
+
+    @Ensures("result >= 0")
     public int getKey() {
         if ( ! isLeaf() )
             throw new ReviewedStingException("Cannot get key of non-leaf node");
@@ -93,10 +128,11 @@ public class StratNode<T extends SetOfStates> implements Iterable<StratNode<T>>
             return key;
     }
 
-    protected Map<String, StratNode<T>> getSubnodes() {
+    protected Map<Object, StratNode<T>> getSubnodes() {
         return subnodes;
     }
 
+    @Ensures("result >= 0")
     public int size() {
         if ( isLeaf() )
             return 1;
@@ -109,9 +145,19 @@ public class StratNode<T extends SetOfStates> implements Iterable<StratNode<T>>
         return stratifier;
     }
 
-    public boolean isLeaf() { return stratifier == null; }
+    /**
+     * @return true if this node is a leaf
+     */
+    public boolean isLeaf() {
+        return stratifier == null;
+    }
 
+    /**
+     * Returns an iterator over this node and all subnodes including internal and leaf nodes
+     * @return
+     */
     @Override
+    @Ensures("result != null")
     public Iterator<StratNode<T>> iterator() {
         return new StratNodeIterator<T>(this);
     }
diff --git a/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStates.java b/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStates.java
index 7f1c75fa9..b6ee7d807 100644
--- a/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStates.java
+++ b/public/java/src/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStates.java
@@ -50,9 +50,9 @@ public class StratificationStates<T extends SetOfStates> {
             return new StratNode<T>();
         } else {
             // we are in the middle of the tree
-            final Collection<String> states = first.getAllStates();
-            final LinkedHashMap<String, StratNode<T>> subNodes = new LinkedHashMap<String, StratNode<T>>(states.size());
-            for ( final String state : states ) {
+            final Collection<Object> states = first.getAllStates();
+            final LinkedHashMap<Object, StratNode<T>> subNodes = new LinkedHashMap<Object, StratNode<T>>(states.size());
+            for ( final Object state : states ) {
                 // have to copy because poll modifies the queue
                 final Queue<T> copy = new LinkedList<T>(strats);
                 subNodes.put(state, buildStratificationTree(copy));
@@ -64,19 +64,38 @@ public class StratificationStates<T extends SetOfStates> {
     public int getNStates() {
         return root.size();
     }
-    
+
     public StratNode<T> getRoot() {
         return root;
     }
     
-    public int getKey(final List<String> states) {
+    public int getKey(final List<Object> states) {
         return root.find(states, 0);
     }
 
+    public Set<Integer> getKeys(final List<List<Object>> allStates) {
+        final HashSet<Integer> keys = new HashSet<Integer>();
+        root.find(allStates, 0, keys);
+        return keys;
+    }
+    
     private void assignKeys(final StratNode<T> root, int key) {
         for ( final StratNode<T> node : root ) {
             if ( node.isLeaf() )
                 node.setKey(key++);
         }
     }
+    
+    public static List<List<Object>> combineStates(final List<Object> first, final List<Object> second) {
+        List<List<Object>> combined = new ArrayList<List<Object>>(first.size());
+        for ( int i = 0; i < first.size(); i++ ) {
+            final Object firstI = first.get(i);
+            final Object secondI = second.get(i);
+            if ( firstI.equals(secondI) ) 
+                combined.add(Collections.singletonList(firstI));
+            else 
+                combined.add(Arrays.asList(firstI, secondI));
+        }
+        return combined;
+    }
 }
diff --git a/public/java/src/org/broadinstitute/sting/utils/Utils.java b/public/java/src/org/broadinstitute/sting/utils/Utils.java
index 130a7fa2f..f91066b0c 100755
--- a/public/java/src/org/broadinstitute/sting/utils/Utils.java
+++ b/public/java/src/org/broadinstitute/sting/utils/Utils.java
@@ -25,6 +25,7 @@
 
 package org.broadinstitute.sting.utils;
 
+import com.google.java.contract.Requires;
 import net.sf.samtools.SAMFileHeader;
 import net.sf.samtools.SAMProgramRecord;
 import net.sf.samtools.util.StringUtil;
@@ -710,4 +711,36 @@ public class Utils {
         }
         return list;
     }
+
+    /**
+     * Returns the number of combinations represented by this collection
+     * of collection of options.
+     *
+     * For example, if this is [[A, B], [C, D], [E, F, G]] returns 2 * 2 * 3 = 12
+     *
+     * @param options
+     * @param <T>
+     * @return
+     */
+    @Requires("options != null")
+    public static <T> int nCombinations(final Collection<T>[] options) {
+        int nStates = 1;
+        for ( Collection<T> states : options ) {
+            nStates *= states.size();
+        }
+        return nStates;
+    }
+
+    @Requires("options != null")
+    public static <T> int nCombinations(final List<List<T>> options) {
+        if ( options.isEmpty() )
+            return 0;
+        else {
+            int nStates = 1;
+            for ( Collection<T> states : options ) {
+                nStates *= states.size();
+            }
+            return nStates;
+        }
+    }
 }
diff --git a/public/java/test/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStatesUnitTest.java b/public/java/test/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStatesUnitTest.java
index 946aef4a9..d6291b812 100644
--- a/public/java/test/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStatesUnitTest.java
+++ b/public/java/test/org/broadinstitute/sting/gatk/walkers/varianteval/stratifications/StratificationStatesUnitTest.java
@@ -30,6 +30,7 @@ package org.broadinstitute.sting.gatk.walkers.varianteval.stratifications;
 
 
 import org.broadinstitute.sting.BaseTest;
+import org.broadinstitute.sting.utils.Utils;
 import org.testng.Assert;
 import org.testng.annotations.BeforeClass;
 import org.testng.annotations.DataProvider;
@@ -51,46 +52,58 @@ public class StratificationStatesUnitTest extends BaseTest {
     // --------------------------------------------------------------------------------
 
     private class StratificationStatesTestProvider extends TestDataProvider {
-        final List<List<Integer>> allStates;
+        final List<List<Object>> allStates = new ArrayList<List<Object>>();
         final List<ListAsSetOfStates> asSetOfStates = new ArrayList<ListAsSetOfStates>();
         final int nStates;
         
         public StratificationStatesTestProvider(final List<Integer> ... allStates) {
             super(StratificationStatesTestProvider.class);
-            this.allStates = Arrays.asList(allStates);
+            
+            for ( List<Integer> states : allStates ) {
+                this.allStates.add(new ArrayList<Object>(states));
+            }
 
-            int nStates = 1;
-            for ( List<Integer> states : this.allStates ) { 
-                nStates *= states.size();
+            for ( List<Object> states : this.allStates ) { 
                 asSetOfStates.add(new ListAsSetOfStates(states));
             }
-            this.nStates = nStates;
-        }
-//        private String getName() {
-//            return String.format("probs=%s expectedRegions=%s", Utils.join(",", probs), Utils.join(",", expectedRegions));
-//        }
+            this.nStates = Utils.nCombinations(allStates);
 
+            setName(getName());
+        }
+
+        private String getName() {
+            StringBuilder b = new StringBuilder();
+            int c = 1;
+            for ( List<Object> state : allStates )
+                b.append(String.format("%d = [%s] ", c++, Utils.join(",", state)));
+            return b.toString();
+        }
+        
         public List<ListAsSetOfStates> getStateSpaceList() {
             return asSetOfStates;
         }
         
-        public Queue<List<String>> getAllCombinations() {
-            return getAllCombinations(new LinkedList<List<Integer>>(allStates));
+        public Queue<List<Object>> getAllCombinations() {
+            return getAllCombinations(new LinkedList<List<Object>>(allStates));
         }
 
-        private Queue<List<String>> getAllCombinations(Queue<List<Integer>> states) {
+        private Queue<List<Object>> getAllCombinations(Queue<List<Object>> states) {
             if ( states.isEmpty() ) 
-                return new LinkedList<List<String>>();
+                return new LinkedList<List<Object>>();
             else {
-                List<Integer> head = states.poll();
-                Queue<List<String>> substates = getAllCombinations(states);
-                Queue<List<String>> newStates = new LinkedList<List<String>>();
-                for ( int e : head) {
-                    for ( List<String> state : substates ) {
-                        List<String> newState = new LinkedList<String>();
-                        newState.add(Integer.toString(e));
-                        newState.addAll(state);
-                        newStates.add(newState);
+                List<Object> head = states.poll();
+                Queue<List<Object>> substates = getAllCombinations(states);
+                Queue<List<Object>> newStates = new LinkedList<List<Object>>();
+                for ( final Object e : head) {
+                    if ( substates.isEmpty() ) {
+                        newStates.add(new LinkedList<Object>(Collections.singleton(e)));
+                    } else {
+                        for ( final List<Object> state : substates ) {
+                            List<Object> newState = new LinkedList<Object>();
+                            newState.add(e);
+                            newState.addAll(state);
+                            newStates.add(newState);
+                        }
                     }
                 }
                 return newStates;
@@ -99,16 +112,14 @@ public class StratificationStatesUnitTest extends BaseTest {
     }
 
     private class ListAsSetOfStates implements SetOfStates {
-        final List<String> integers;
+        final List<Object> integers;
 
-        private ListAsSetOfStates(final List<Integer> integers) {
-            this.integers = new ArrayList<String>(integers.size());
-            for ( int i : integers )
-                this.integers.add(Integer.toString(i));
+        private ListAsSetOfStates(final List<Object> integers) {
+            this.integers = integers;
         }
-
+        
         @Override
-        public List<String> getAllStates() {
+        public List<Object> getAllStates() {
             return integers;
         }
     }
@@ -127,8 +138,8 @@ public class StratificationStatesUnitTest extends BaseTest {
     }
 
     @Test(dataProvider = "StratificationStatesTestProvider")
-    public void testStratificationStatesTestProvider(StratificationStatesTestProvider cfg) {
-        StratificationStates<ListAsSetOfStates> stratificationStates = new StratificationStates<ListAsSetOfStates>(cfg.getStateSpaceList());
+    public void testLeafCount(StratificationStatesTestProvider cfg) {
+        final StratificationStates<ListAsSetOfStates> stratificationStates = new StratificationStates<ListAsSetOfStates>(cfg.getStateSpaceList());
 
         Assert.assertEquals(stratificationStates.getNStates(), cfg.nStates);
         
@@ -138,20 +149,55 @@ public class StratificationStatesUnitTest extends BaseTest {
                 nLeafs++;
         }
         Assert.assertEquals(nLeafs, cfg.nStates, "Unexpected number of leaves");
-        
-        Set<Integer> seenKeys = new HashSet<Integer>(cfg.nStates);
+    }
+
+    @Test(dataProvider = "StratificationStatesTestProvider")
+    public void testKeys(StratificationStatesTestProvider cfg) {
+        final StratificationStates<ListAsSetOfStates> stratificationStates = new StratificationStates<ListAsSetOfStates>(cfg.getStateSpaceList());
+        final Set<Integer> seenKeys = new HashSet<Integer>(cfg.nStates);
         for ( final StratNode node : stratificationStates.getRoot() ) {
             if ( node.isLeaf() ) {
                 Assert.assertFalse(seenKeys.contains(node.getKey()), "Already seen the key");
                 seenKeys.add(node.getKey());
             }
         }
+    }
 
-        seenKeys.clear();
-        for ( List<String> state : cfg.getAllCombinations() ) {
+    @Test(dataProvider = "StratificationStatesTestProvider")
+    public void testFindSingleKeys(StratificationStatesTestProvider cfg) {
+        final StratificationStates<ListAsSetOfStates> stratificationStates = new StratificationStates<ListAsSetOfStates>(cfg.getStateSpaceList());
+        final Set<Integer> seenKeys = new HashSet<Integer>(cfg.nStates);
+        for ( List<Object> state : cfg.getAllCombinations() ) {
             final int key = stratificationStates.getKey(state);
             Assert.assertFalse(seenKeys.contains(key), "Already saw state mapping to this key");
             seenKeys.add(key);
         }
     }
+
+    @Test(dataProvider = "StratificationStatesTestProvider")
+    public void testFindMultipleKeys(StratificationStatesTestProvider cfg) {
+        final StratificationStates<ListAsSetOfStates> stratificationStates = new StratificationStates<ListAsSetOfStates>(cfg.getStateSpaceList());
+        final List<List<Object>> states = new ArrayList<List<Object>>(cfg.allStates);
+        final Set<Integer> keys = stratificationStates.getKeys(states);
+        Assert.assertEquals(keys.size(), cfg.nStates, "Find all states didn't find all of the expected unique keys");
+
+        final Queue<List<Object>> combinations = cfg.getAllCombinations();
+        while ( ! combinations.isEmpty() ) {
+            List<Object> first = combinations.poll();
+            List<Object> second = combinations.peek();
+            if ( second != null ) {
+                List<List<Object>> combined = StratificationStates.combineStates(first, second);
+                int nExpectedKeys = Utils.nCombinations(combined);
+
+                final int key1 = stratificationStates.getKey(first);
+                final int key2 = stratificationStates.getKey(second);
+                final Set<Integer> keysCombined = stratificationStates.getKeys(combined);
+            
+                Assert.assertTrue(keysCombined.contains(key1), "couldn't find key in data set");
+                Assert.assertTrue(keysCombined.contains(key2), "couldn't find key in data set");
+                
+                Assert.assertEquals(keysCombined.size(), nExpectedKeys);
+            }
+        }
+    }
 }
\ No newline at end of file