code: void copy(State source)
      {
          this.ref = source.ref;
          this.score = source.score;
          this.previous = source.previous;
          this.buffer = source.buffer;
          this.top0 = source.top0;
          this.top1 = source.top1;
          this.stack = source.stack;
          this.last_action = source.last_action;
          this.heads = source.heads;
          this.deprels = source.deprels;
          this.left_most_child = source.left_most_child;
          this.right_most_child = source.right_most_child;
          this.left_2nd_most_child = source.left_2nd_most_child;
          this.right_2nd_most_child = source.right_2nd_most_child;
          this.nr_left_children = source.nr_left_children;
          this.nr_right_children = source.nr_right_children;
      }
docs: 克隆一个状态到自己
  @param source 源状态