@Override
	public void add_child(Sentence_tree child){
		Sentence_tree[] new_children = new Sentence_tree[this.children.length + 1];
		for(int i = 0; i < this.children.length; i++){
			new_children[i] = this.children[i];
		}
		new_children[this.children.length] = child;
		child.parent = this;
		this.children = new_children;
	}