code: public TupleCombiner addOnceTuple( TupleRef tupleRef)
      {
      if( tupleRef != null)
        {
        if( tupleRef.size() != getTupleSize())
          {
          throw new IllegalArgumentException( "Once-only tuple=" + tupleRef + " has size=" + tupleRef.size() + ", expected size=" + getTupleSize());
          }
        onceTuples_.add( tupleRef);
        }
      
      return this;
      }
docs: Adds a once-only tuple to this combination.