See for example the Prolog implementation in PAIP: https://github.com/norvig/paip-lisp/blob/main/docs/chapter11.md https://github.com/norvig/paip-lisp/blob/mai

See for example the Prolog implementation in PAIP: https://github.com/norvig/paip-lisp/blob/main/docs/chapter11.md https://github.com/norvig/paip-lisp/blob/main/docs/chapter11... But I would think the convention is quite a bit older than Common Lisp and earlier Lisp programs should have used something like that to notate variables in special sublanguages (especially in simple Prolog variants or in Rule-based systems).