_removeNode(node) {
    for (let key in node.occurrences) {
      for (let i = 0; i < node.occurrences[key].length; i++) {
      if (!this._map[key]) break;
        let index = this._map[key].indexOf(node.occurrences[key][i]);
        if (index > -1) {
          this._map[key].splice(index, 1);
        }

        // Delete empty buckets
        if (!this._map[key].length) {
          delete this._map[key];
        }
      }

      delete node.occurrences[key];
    }
  }
Removes all occurrences of a given node.

@private

@param {Node} node The node to remove