func (s *VecStack) Peek() (value *Vec, exists bool) {
	exists = false
	if s.size > 0 {
		value = s.top.value
		exists = true
	}
	return
}
func (s *VecStack) Peek() (value *Vec, exists bool) {
	exists = false
	if s.size > 0 {
		value = s.top.value
		exists = true
	}
	return
}
// Peek returns the topmost item without removing it from the stack.