func (l *List) Swap(x, y uint) bool {
	l.key.Lock()
	defer l.key.Unlock()
	return l.swap(x, y)
}
func (l *List) Swap(x, y uint) bool {
	l.key.Lock()
	defer l.key.Unlock()
	return l.swap(x, y)
}
// Swap switches the values that are stored at positions `x` and `y`