`(scheme aux)` is a clever idea. I don't think define-syntax is enough to guarantee keyword hygiene, though.

If you `(define-syntax <>)`, it's still possible to write `(let ((<> 1)) <>)` without errors. That makes it possible for something inside the `let` scope to unhygenically insert a `<>` into a macro that uses `cut` or `chain` internally. An identifier macro would prevent this.