code: func Forbidden(field *Path, detail string) *Error {
  	return &Error{ErrorTypeForbidden, field.String(), "", detail}
  }
docs: // Forbidden returns a *Error indicating "forbidden".  This is used to
  // report valid (as per formatting rules) values which would be accepted under
  // some conditions, but which are not permitted by current conditions (e.g.
  // security policy).