In addition to the sentinel/in-band replies, they also put properties on that symbol: https://github.com/emacs-mirror/emacs/blob/81b5131e11fa96d32a1718c6e2ebf40

In addition to the sentinel/in-band replies, they also put properties on that symbol: https://github.com/emacs-mirror/emacs/blob/81b5131e11fa96d32a1718c6e2ebf40994d7d281/lisp/emacs-lisp/generator.el#L569 https://github.com/emacs-mirror/emacs/blob/81b5131e11fa96d32... ( https://www.gnu.org/software/emacs/manual/html_node/elisp/Symbol-Plists.html https://www.gnu.org/software/emacs/manual/html_node/elisp/Sy... )