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