        this.$.tree
        return this.items;
      }

      _makeButton(tag, id, index) {
        let aButton = document.createElement('button');
        aButton.dataset.index = index;
        let aTag = document.createElement('span');
        aTag.className = 'tag';
        aTag.textContent = tag;
        let aId = document.createElement('span');
        aId.className = 'id';
        aId.textContent = id;
        aButton.appendChild(aTag);
        aButton.appendChild(aId);
        return aButton;
      }