func (l *List) IsEmpty() bool {
	l.key.RLock()
	defer l.key.RUnlock()
	return 0 == len(l.underlyer)
}
func (l *List) IsEmpty() bool {
	l.key.RLock()
	defer l.key.RUnlock()
	return 0 == len(l.underlyer)
}
// IsEmpty tests to see if this List has any elements present.