--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/main/gov/nasa/jpf/util/BitSetN.java	Fri Jan 23 10:14:01 2015 -0800
@@ -0,0 +1,122 @@
+/*
+ * Copyright (C) 2014, United States Government, as represented by the
+ * Administrator of the National Aeronautics and Space Administration.
+ * All rights reserved.
+ *
+ * The Java Pathfinder core (jpf-core) platform is licensed under the
+ * Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ * 
+ *        http://www.apache.org/licenses/LICENSE-2.0. 
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and 
+ * limitations under the License.
+ */
+
+package gov.nasa.jpf.util;
+
+import java.util.BitSet;
+import java.util.NoSuchElementException;
+
+/**
+ * a FixedBitSet implementation that is based on java.util.BitSet
+ */
+public class BitSetN extends BitSet implements FixedBitSet {
+  
+  class SetBitIterator implements IntIterator {
+    int cur = 0;
+    int nBits;
+    int cardinality;  // <2do> this should be lifted since it makes the iterator brittle
+    
+    SetBitIterator (){
+      cardinality = cardinality();
+    }
+    
+    @Override
+    public void remove() {
+      if (cur >0){
+        clear(cur-1);
+      }
+    }
+
+    @Override
+    public boolean hasNext() {
+      return nBits < cardinality;
+    }
+
+    @Override
+    public int next() {
+      if (nBits < cardinality){
+        int idx = nextSetBit(cur);
+        if (idx >= 0){
+          nBits++;
+          cur = idx+1;
+        }
+        return idx;
+        
+      } else {
+        throw new NoSuchElementException();
+      }
+    }
+  }
+  
+  
+  public BitSetN (int nBits){
+    super(nBits);
+  }
+  
+  @Override
+  public FixedBitSet clone() {
+    return (FixedBitSet) super.clone();
+  }
+
+  @Override
+  public int capacity() {
+    return size();
+  }
+
+
+  @Override
+  public void hash (HashData hd){
+    long[] data = toLongArray();
+    for (int i=0; i<data.length; i++){
+      hd.add(data[i]);
+    }
+  }
+
+  
+  //--- IntSet interface
+  @Override
+  public boolean add(int i) {
+    if (get(i)) {
+      return false;
+    } else {
+      set(i);
+      return true;
+    }
+  }
+
+  @Override
+  public boolean remove(int i) {
+    if (get(i)) {
+      clear(i);
+      return true;
+    } else {
+      return false;
+    }
+  }
+
+  @Override
+  public boolean contains(int i) {
+    return get(i);
+  }
+  
+  @Override
+  public IntIterator intIterator() {
+    return new SetBitIterator();
+  }
+  
+}