As is a convenience function that returns err as an Error and true if err is an Error, and err and false otherwise.

Is is a convenience function that returns true if err is a system error.

New is a convenience function that returns a new system error by calling errors.new.

Newf is a convenience function that returns a new system error by calling fmt.Errorf.

Wrap returns a new system error for err.

Error is a system error.