code: function(crop) {
              if(typeof crop === 'undefined') {
                  crop = this._crop;
              }
              crop.usable = !crop.usable;
              this._renderUpdatedPreview(crop);
  
              if (crop.id === this._crop.id) {
                  this._cropUsableToggle.on = crop.usable;
              }
          }
docs: Toggle a specified crops usable field, or the current crop if left out
  @param crop