private void findLastUsage_BFS() {
		// TODO Auto-generated method stub
		List<Unit> first_node = graph.getHeads();
		if(first_node.size()>1){
			return;
		}
		queue.add(first_node.get(0));
		units_already_seen.add(first_node.get(0));
		
		
		while(!queue.isEmpty()){
			Unit u = queue.remove();
			List<Local> vr = sll.getLiveLocalsAfter(u);
			Set<Local> dead_keys = getAllDeadKeys(sll.getLiveLocalsAfter(u));
			//map_keyset.removeAll(dead_keys);			
			if(!dead_keys.isEmpty()){
				result_map.put(u, dead_keys);
			}
			//units_already_seen.add(u);
			for(Unit succ: graph.getSuccsOf(u)){
				if(!units_already_seen.contains(succ)){
					units_already_seen.add(succ);
					queue.add(succ);
				}
			}
			
		}
		
	}