code: func (s *Set) AddIdentity(id identity.Identity) {
  	if !s.All {
  		if s.IDs == nil {
  			s.IDs = make(identSet, 1)
  		}
  		s.IDs[id] = struct{}{}
  	}
  }
docs: // AddIdentity adds a single identity to the set.
  //
  // The receiver must not be nil.