function Stack(head, tail, idx, len) {
    this.tail = tail;
    this.isObject = head && typeof head === 'object';
    this.head = head;
    this.index = idx;
    this.of = len;
  }
Get a value from the context
@method _get
@param {boolean} cur Get only from the current context
@param {array} down An array of each step in the path
@private
@return {string | object}