code: func (l *List) Set(pos uint, val interface{}) bool {
  	l.key.Lock()
  	defer l.key.Unlock()
  	var retval bool
  	count := uint(len(l.underlyer))
  	if pos > count {
  		retval = false
  	} else {
  		l.underlyer[pos] = val
  		retval = true
  	}
  	return retval
  }
docs: // Set updates the value stored at a given position in the List.