code: func (s *Set) Add(x uint32) {
  	v := s.sparse[x]
  	if v < uint32(len(s.dense)) && s.dense[v] == x {
  		return
  	}
  	n := len(s.dense)
  	s.sparse[x] = uint32(n)
  	s.dense = append(s.dense, x)
  }
docs: // Add adds x to the set if it is not already there.