@Override
	public Sentence_tree transform_tree(String keyword1, String keyword2){
		Hashtable<String,Boolean> keyword = new Hashtable<String,Boolean>();
		keyword.put(keyword2, true);
		int index = 0;
		for(index = 0; index < children.length; index++){
			if(!children[index].bfs_name(keyword).isEmpty()) break; // the branch is found.
		}
		this.parent = this.children[index];
		this.parent.root.set_function("root");
		
		this.children[index].add_child(new Sentence_tree(new Node("être","V","aux_pass"))); // add the passive verb "être"
		this.children[index].add_child(this);
		this.root.set_function("suj");
		
		this.remove_child(this.parent);
		
		return this.parent;
	}