docs: function addListener(emitter, event, fn, context, once) {
    if (typeof fn !== 'function') {
      throw new TypeError('The listener must be a function');
    }
  
    var listener = new EE(fn, context || emitter, once)
      , evt = prefix ? prefix + event : event;
  
    if (!emitter._events[evt]) emitter._events[evt] = listener, emitter._eventsCount++;
    else if (!emitter._events[evt].fn) emitter._events[evt].push(listener);
    else emitter._events[evt] = [emitter._events[evt], listener];
  
    return emitter;
  }