code: function once(events, callback, context)
    {
      return onListeners( this, events, callback, context, EventNode.Types.Once );
    }
docs: Listens for the first of the given events to be triggered and invokes the
  callback once.
  
  @method once
  @memberof Rekord.Eventful#
  @param {String|Array} events -
  The event or events to listen to.
  @param {Function} callback -
  The function to invoke when any of the events are invoked.
  @param {Object} [context] -
  The value of `this` when the callback is invoked. If not specified, the
  reference of the object this function exists on will be `this`.
  @return {Function} -
  A function to invoke to stop listening to all of the events given.
   
  Listens for the first of the given events to be triggered and invokes the
  callback once.
  
  @method $once
  @memberof Rekord.Eventful$#
  @param {String|Array} events -
  The event or events to listen to.
  @param {Function} callback -
  The function to invoke when any of the events are invoked.
  @param {Object} [context] -
  The value of `this` when the callback is invoked. If not specified, the
  reference of the object this function exists on will be `this`.
  @return {Function} -
  A function to invoke to stop listening to all of the events given.