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