code: func (s *UserService) FindUser(ctx context.Context, filter platform.UserFilter) (*platform.User, error) {
  	users, n, err := s.FindUsers(ctx, filter)
  	if err != nil {
  		return nil, &platform.Error{
  			Op:  s.OpPrefix + platform.OpFindUser,
  			Err: err,
  		}
  	}
  
  	if n == 0 {
  		return nil, &platform.Error{
  			Code: platform.ENotFound,
  			Op:   s.OpPrefix + platform.OpFindUser,
  			Msg:  "no results found",
  		}
  	}
  
  	return users[0], nil
  }
docs: // FindUser returns the first user that matches filter.