code: func NotSupported(field *Path, value interface{}, validValues []string) *Error {
  	detail := ""
  	if validValues != nil && len(validValues) > 0 {
  		detail = "supported values: " + strings.Join(validValues, ", ")
  	}
  	return &Error{ErrorTypeNotSupported, field.String(), value, detail}
  }
docs: // NotSupported returns a *Error indicating "unsupported value".
  // This is used to report unknown values for enumerated fields (e.g. a list of
  // valid values).