code: func (v PortRangeVar) Set(s string) error {
  	if _, err := utilnet.ParsePortRange(s); err != nil {
  		return fmt.Errorf("%q is not a valid port range: %v", s, err)
  	}
  	if v.Val == nil {
  		// it's okay to panic here since this is programmer error
  		panic("the string pointer passed into PortRangeVar should not be nil")
  	}
  	*v.Val = s
  	return nil
  }
docs: // Set sets the flag value