code: func (s *SegmentStack) Pop() (int64, bool) {
  	s.Lock()
  	defer s.Unlock()
  	if s.Len() == 0 {
  		return rootSegment, false
  	}
  	id := s.s[s.Len()-1]
  	s.s = s.s[:s.Len()-1]
  	return id, true
  }
docs: // Pop pops a segment id off of the segment stack. It returns false if the stack is empty.