code: func (s *sequences) next(seq string) int {
  	if s.s == nil {
  		s.s = make(map[string]int, 1)
  	}
  	v := s.s[seq] + 1
  	s.s[seq] = v
  	return v
  }
docs: // next returns the next number in the given sequence.
  //
  // Sequences start with 1.