Re: emacs-29 additional key binding suggestions

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel@gnu.org
> Date: Tue, 25 Apr 2023 17:19:47 +0200
> 
> +*** 'browse-url-of-dired-file' can now call the secondary browser.
> +When invoked with a non-nil prefix arg, this will now call
> +'browse-url-secondary-browser-function' instead of the default

"Non-nil prefix arg"? what's that? is there a "nil prefix arg"?