code: func (s *OrderedSet) Add(i string) bool {
  	if _, ok := s.m[i]; ok {
  		return false
  	}
  
  	s.s = append(s.s, i)
  	s.m[i] = len(s.s) - 1
  
  	return true
  }
docs: // Add adds the given element "i" to the ordered set, unless the element is
  // already present. It returns whether or not the element was added.