code: function(event, fn) {
          // Get handle to event, creating it if necessary
          var callbacks = this._callbacks[event];
          if (!callbacks) {
              callbacks = this._callbacks[event] = (handlers[event] ||
                  (handlers[event] = {}))[this[0]] = [];
              callbacks.context = this;
              callbacks.depth = 0;
          }
          // Push to callback array
          callbacks.push(fn);
      }
docs: Add a function to the list of callbacks for an event