docs: function(prop, current) {
                      if (prop in this._data) {
                          // Delete the property from `_data` and the Map
                          // as long as it isn't part of the Map's prototype.
                          delete this._data[prop];
                          if (!(prop in this.constructor.prototype)) {
                              delete this[prop];
                          }
                          // Let others now this property has been removed.
                          this._triggerChange(prop, "remove", undefined, current);
  
                      }
                  }