getActiveNodes = id => {
      if (map.has(id)) return map.get(id) || [];

      const node = this._nodesById.get(id);
      if (!node) throw new Error(`No such node ${id}`);
      if (typeof node.parent === 'number') {
        const array = getActiveNodes(node.parent).concat([id]);
        map.set(id, array);
        return array;
      } else {
        return [id];
      }
    }
@param {number} id @return {Array<number>}