private static BitSet mergeProhibitedVariables(BitSet leftSet, BitSet rightSet) {
    if (leftSet.equals(rightSet)) {
      return leftSet;
    }
    BitSet prohibited = ((BitSet)leftSet.clone());
    prohibited.or(rightSet);
    return prohibited;
  }
private static BitSet mergeProhibitedVariables(BitSet leftSet, BitSet rightSet)