code: func WithUser(username string, org string) UsersOption {
  	return func(mgr *UsersOptions) error {
  		if mgr.users != nil {
  			return errors.New("already initialized")
  		}
  		mgr.users = make(map[string]msp.SigningIdentity)
  		mgr.users[username] = mspmocks.NewMockSigningIdentity(username, org)
  		return nil
  	}
  }
docs: // WithUser option