@Override
	public Sentence_tree bfs_function(String function){
		LinkedList<Sentence_tree> queue = new LinkedList<Sentence_tree>();
		queue.add(this);
		while(!queue.isEmpty()){
			Sentence_tree current = queue.pop();
			if(current.root.function.equals(function)){
				return current;
			}
			for(int index = 0; index < current.children.length; index++){
				queue.addLast(current.children[index]);
			}
		}
		return null;
	}