create (node) {
    let values = node.get(this._property)
    if (!isArray(values)) {
      values = [values]
    }
    values.forEach(value => {
      this.index.set([value, node.id], node)
    })
  }
Called when a node has been created.

    Override this in subclasses for customization.

    @private
    @param {Node} node