code: func (s *Set) AddGroup(group string) {
  	if !s.All {
  		if s.Groups == nil {
  			s.Groups = make(groupSet, 1)
  		}
  		s.Groups[group] = struct{}{}
  	}
  }
docs: // AddGroup adds a single group to the set.
  //
  // The receiver must not be nil.