diff --git a/com.ibm.wala.core/src/com/ibm/wala/util/intset/IntSetUtil.java b/com.ibm.wala.core/src/com/ibm/wala/util/intset/IntSetUtil.java
index b368649ec..773a91c86 100644
--- a/com.ibm.wala.core/src/com/ibm/wala/util/intset/IntSetUtil.java
+++ b/com.ibm.wala.core/src/com/ibm/wala/util/intset/IntSetUtil.java
@@ -20,7 +20,26 @@ import com.ibm.wala.util.debug.Trace;
  */
 public class IntSetUtil {
 
-  private static MutableIntSetFactory defaultIntSetFactory = new MutableSharedBitVectorIntSetFactory();
+  public static final String INT_SET_FACTORY_CONFIG_PROPERTY_NAME = "com.ibm.wala.mutableIntSetFactory";
+  
+  private static MutableIntSetFactory defaultIntSetFactory;
+  
+  static {
+	  MutableIntSetFactory defaultFactory = new MutableSharedBitVectorIntSetFactory();
+	  if (System.getProperties().containsKey(INT_SET_FACTORY_CONFIG_PROPERTY_NAME)) {
+		  try {
+		    Class intSetFactoryClass = Class.forName(System.getProperty(INT_SET_FACTORY_CONFIG_PROPERTY_NAME));
+		    MutableIntSetFactory intSetFactory = (MutableIntSetFactory) intSetFactoryClass.newInstance();
+		    setDefaultIntSetFactory(intSetFactory);
+		  } catch (Exception e) {
+			Trace.println("Cannot use int set factory " + System.getProperty(INT_SET_FACTORY_CONFIG_PROPERTY_NAME));
+			setDefaultIntSetFactory(defaultFactory);
+		  }
+	  } else {
+		setDefaultIntSetFactory(defaultFactory);
+	  }
+	  assert defaultIntSetFactory != null;
+  }
   
   public static MutableIntSet make() {
     return defaultIntSetFactory.make();
@@ -44,6 +63,12 @@ public class IntSetUtil {
       return BimodalMutableIntSet.makeCopy(set);
     } else if (set instanceof MutableSharedBitVectorIntSet) {
       return new MutableSharedBitVectorIntSet((MutableSharedBitVectorIntSet) set);
+    } else if (set instanceof SemiSparseMutableIntSet) {
+      return new SemiSparseMutableIntSet((SemiSparseMutableIntSet) set);
+    } else if (set instanceof DebuggingMutableIntSet) {
+    	MutableIntSet pCopy = makeMutableCopy(((DebuggingMutableIntSet)set).primaryImpl);
+    	MutableIntSet sCopy = makeMutableCopy(((DebuggingMutableIntSet)set).secondaryImpl);
+    	return new DebuggingMutableIntSet(pCopy, sCopy);
     } else {
       Assertions.UNREACHABLE(set.getClass().toString());
       return null;
diff --git a/com.ibm.wala.core/src/com/ibm/wala/util/intset/MutableSharedBitVectorIntSet.java b/com.ibm.wala.core/src/com/ibm/wala/util/intset/MutableSharedBitVectorIntSet.java
index e2e3d08dc..5fd7c706a 100644
--- a/com.ibm.wala.core/src/com/ibm/wala/util/intset/MutableSharedBitVectorIntSet.java
+++ b/com.ibm.wala.core/src/com/ibm/wala/util/intset/MutableSharedBitVectorIntSet.java
@@ -335,6 +335,8 @@ public class MutableSharedBitVectorIntSet implements MutableIntSet {
       return that.sameValue(makeSparseCopy());
     } else if (that instanceof BitVectorIntSet) {
       return sameValue((BitVectorIntSet) that);
+    } else if (that instanceof SemiSparseMutableIntSet) {
+        return that.sameValue(this);   	
     } else {
       Assertions.UNREACHABLE("unexpected class " + that.getClass());
       return false;
diff --git a/com.ibm.wala.core/src/com/ibm/wala/util/intset/SemiSparseMutableIntSet.java b/com.ibm.wala.core/src/com/ibm/wala/util/intset/SemiSparseMutableIntSet.java
index fa0fc6196..354060cd9 100644
--- a/com.ibm.wala.core/src/com/ibm/wala/util/intset/SemiSparseMutableIntSet.java
+++ b/com.ibm.wala.core/src/com/ibm/wala/util/intset/SemiSparseMutableIntSet.java
@@ -22,6 +22,14 @@ public class SemiSparseMutableIntSet implements MutableIntSet {
 
   private int sparseInsertCount = 0;
 
+  public SemiSparseMutableIntSet() {
+    // this space intentionally left blank
+  }
+
+  public SemiSparseMutableIntSet(SemiSparseMutableIntSet set) {
+	copySet(set);
+  }
+
   private void fixAfterSparseInsert() {
     if (sparseInsertCount++ > SPARSE_INSERT_THRESHOLD) {
       sparseInsertCount = 0;
@@ -58,7 +66,7 @@ public class SemiSparseMutableIntSet implements MutableIntSet {
         }
 
         if (maxOffset != -1) {
-          densePart = new OffsetBitVector(maxOffset, maxMax);
+          densePart = new OffsetBitVector(maxOffset, maxMax-maxOffset);
           sparseBits = sparsePart.intIterator();
           while ((sparseBits.next()) != maxOffset)
             ;
@@ -215,7 +223,7 @@ public class SemiSparseMutableIntSet implements MutableIntSet {
 
       public int next() {
         int next = densePart.nextSetBit(i + 1);
-        i = next + 1;
+        i = next;
         return next;
       }
     }
@@ -488,4 +496,13 @@ public class SemiSparseMutableIntSet implements MutableIntSet {
 
     return change;
   }
+  
+  public String toString() {
+	  StringBuffer sb = new StringBuffer("[");
+	  if (densePart !=  null) {
+		  sb.append("densePart: ").append(densePart.toString()).append(" ");
+	  }
+	  sb.append("sparsePart: ").append(sparsePart.toString()).append("]");
+	  return sb.toString();
+  }
 }