docs: function(event, callback) {
          //  To learn how the event system functions, see the comments for Crafty._callbackMethods
          //optimization for 1 entity
          if (this.length === 1) {
              this._bindCallback(event, callback);
          } else {
              for (var i = 0; i < this.length; i++) {
                  var e = entities[this[i]];
                  if (e) {
                      e._bindCallback(event, callback);
                  }
              }
          }
          return this;
      }