docs: function appendListener(event, listener, once) {
  
      this.events[event] = this.events[event] || [];
      this.events[event].push({
        "listener": listener,
        "once": !!once
      });
  
      this.emit('newListener', event, listener);
  
      if (this.events[event].length > this.maxListeners) {
  
        console.warn('warning: possible EventEmitter memory leak detected. ',
          this.events[event].length, ' listeners added. ',
          'Use emitter.setMaxListeners() to increase limit. ',
          this);
  
      }
  
    }