lib/event-emitter.js
function once(event, listener) {
  function g() {
    this.removeListener(event, g);
    listener.apply(this, arguments);
  };
  this.on(event, g);
  return this;
}
function once(event, listener) {
  function g() {
    this.removeListener(event, g);
    listener.apply(this, arguments);
  };
  this.on(event, g);
  return this;
}
function once ( event , listener ) { function g ( ) { this . removeListener ( event , g ) ; listener . apply ( this , arguments ) ; } ; this . on ( event , g ) ; return this ; }
Registers a listener to be invoked once.

@param event The event name.
@param listener The listener function.
Registers a listener to be invoked once .