code: function(eventName, callback) {
          var allCallbacks = lib$rsvp$events$$callbacksFor(this), callbacks, index;
  
          if (!callback) {
            allCallbacks[eventName] = [];
            return;
          }
  
          callbacks = allCallbacks[eventName];
  
          index = lib$rsvp$events$$indexOf(callbacks, callback);
  
          if (index !== -1) { callbacks.splice(index, 1); }
        }
docs: You can use `off` to stop firing a particular callback for an event:
  
  ```javascript
  function doStuff() { // do stuff! }
  object.on('stuff', doStuff);
  
  object.trigger('stuff'); // doStuff will be called
  
  Unregister ONLY the doStuff callback
  object.off('stuff', doStuff);
  object.trigger('stuff'); // doStuff will NOT be called
  ```
  
  If you don't pass a `callback` argument to `off`, ALL callbacks for the
  event will not be executed when the event fires. For example:
  
  ```javascript
  var callback1 = function(){};
  var callback2 = function(){};
  
  object.on('stuff', callback1);
  object.on('stuff', callback2);
  
  object.trigger('stuff'); // callback1 and callback2 will be executed.
  
  object.off('stuff');
  object.trigger('stuff'); // callback1 and callback2 will not be executed!
  ```
  
  @method off
  @for RSVP.EventTarget
  @private
  @param {String} eventName event to stop listening to
  @param {Function} callback optional argument. If given, only the function
  given will be removed from the event's callback queue. If no `callback`
  argument is given, all callbacks will be removed from the event's callback
  queue.