code: func (s *parentStack) Pop() *treapNode {
  	if s.index == 0 {
  		return nil
  	}
  
  	s.index--
  	if s.index < staticDepth {
  		node := s.items[s.index]
  		s.items[s.index] = nil
  		return node
  	}
  
  	node := s.overflow[s.index-staticDepth]
  	s.overflow[s.index-staticDepth] = nil
  	return node
  }
docs: // Pop removes the top item from the stack.  It returns nil if the stack is
  // empty.