It's common for Emacs commands that prompt for a numeric argument in
the minibuffer to also support reading that numeric argument via a
prefix argument. (See goto-char, for example.).
rfc-mode.el (rfc-mode-read): Read a prefix numeric argument and use
it as the "number" argument; prompt in the minibuffer if no prefix
argument was used.
It's common for Emacs commands that prompt for a numeric argument in the minibuffer to also support reading that numeric argument via a prefix argument. (See
goto-char
, for example.).