f64/d2/vec_stack.go
func (s *VecStack) PopLast() (value *Vec) {
	if lastElem := s.popLast(s.top); s.size >= 2 && lastElem != nil {
		return lastElem.value
	}
	return nil
}
func (s *VecStack) PopLast() (value *Vec) {
	if lastElem := s.popLast(s.top); s.size >= 2 && lastElem != nil {
		return lastElem.value
	}
	return nil
}
// PopLast removes the bottommost item.
//
// PopLast does nothing if the stack does not contain at least 2 items.