code: func (s *Server) logf(format string, args ...interface{}) {
  	if s.ErrorLog != nil {
  		s.ErrorLog.Printf(format, args...)
  	} else {
  		log.Printf(format, args...)
  	}
  }
docs: // logf logs a message using the server's ErrorLog logger, or the log package
  // standard logger, if ErrorLog is nil.