code: public TupleCombiner cloneOf()
      {
      TupleCombiner other = new TupleCombiner();
      other.setTupleSize( getTupleSize());
      other.setIncludedVars( new HashSet<VarNamePattern>( getIncludedVars()));
      other.setExcludedVars( new HashSet<VarNamePattern>( getExcludedVars()));
      other.setOnceTuples( new HashSet<TupleRef>( IteratorUtils.toList( getOnceTuples())));
      return other;
      }
docs: Returns a copy of this object.