stack.go
func (stack *Stack) Peek() (interface{}, bool) {
	stack.key.RLock()
	defer stack.key.RUnlock()
	return stack.underlyer.PeekFront()
}
func (stack *Stack) Peek() (interface{}, bool) {
	stack.key.RLock()
	defer stack.key.RUnlock()
	return stack.underlyer.PeekFront()
}
// Peek returns the entry at the top of the Stack without removing it.