code: func (l *State) serviceToken(id string) string {
  	var token string
  	if s := l.services[id]; s != nil {
  		token = s.Token
  	}
  	if token == "" {
  		token = l.tokens.UserToken()
  	}
  	return token
  }
docs: // serviceToken returns an ACL token associated with a service.
  // This method is not synchronized and the lock must already be held.