code: func UserPasswd(server, preferredUser string) (user, passwd string, err error) {
  	user, passwd, err = userPasswd(server, preferredUser)
  	if err != nil {
  		if preferredUser != "" {
  			err = fmt.Errorf("loading password for %s@%s: %v", preferredUser, server, err)
  		} else {
  			err = fmt.Errorf("loading password for %s: %v", server, err)
  		}
  	}
  	return
  }
docs: // UserPasswd returns the user name and password for authenticating
  // to the named server.  If the user argument is non-empty, UserPasswd
  // restricts its search to passwords for the named user.