code: func (list *LinkedList) PeekBack() (interface{}, bool) {
  	list.key.RLock()
  	defer list.key.RUnlock()
  
  	if list.last == nil {
  		return nil, false
  	}
  	return list.last.payload, true
  }
docs: // PeekBack returns the entry logicall stored at the back of the list without removing it.