code: func (s *VecStack) Pop() (value *Vec) {
  	if s.size > 0 {
  		value, s.top = s.top.value, s.top.next
  		s.size--
  		return
  	}
  	return nil
  }
docs: // Pop removes the topmost item from the stack and return its value.
  //
  // If the stack is empty, Pop returns nil.