stack.go
func (stack *Stack) IsEmpty() bool {
	stack.key.RLock()
	defer stack.key.RUnlock()
	return stack.underlyer == nil || stack.underlyer.IsEmpty()
}
func (stack *Stack) IsEmpty() bool {
	stack.key.RLock()
	defer stack.key.RUnlock()
	return stack.underlyer == nil || stack.underlyer.IsEmpty()
}
// IsEmpty tests the Stack to determine if it is populate or not.