code: func IsConversionError(err error) (bool, error) {
  	e, ok := errs.Cause(err).(ConversionError)
  	if !ok {
  		return false, nil
  	}
  	return true, e
  }
docs: // IsConversionError returns true if the cause of the given error can be
  // converted to an ConversionError, which is returned as the second result.