code: func (s *Set) Add(d Dependency) bool {
  	s.init()
  	s.Lock()
  	defer s.Unlock()
  	if _, ok := s.set[d.String()]; !ok {
  		s.list = append(s.list, d.String())
  		s.set[d.String()] = d
  		return true
  	}
  	return false
  }
docs: // Add adds a new element to the set if it does not already exist.