code: func (l *waiterList) InsertBefore(a, e *Entry) {
  	b := waiterElementMapper{}.linkerFor(a).Prev()
  	waiterElementMapper{}.linkerFor(e).SetNext(a)
  	waiterElementMapper{}.linkerFor(e).SetPrev(b)
  	waiterElementMapper{}.linkerFor(a).SetPrev(e)
  
  	if b != nil {
  		waiterElementMapper{}.linkerFor(b).SetNext(e)
  	} else {
  		l.head = e
  	}
  }
docs: // InsertBefore inserts e before a.