next_ops = new HashSet();
next_ops = new HashSet<SIROperator>();
    public void findPhases(SIROperator top) {	phases.put(top, new Integer(0));	current_ops.add(top);            do {            findNextPhase();            current_ops = next_ops;            next_ops = new HashSet();            number_of_phases++;        } while (current_ops.size() != 0);        }