to invoke it every time I respond to an article.

It won't change the subject if it doesn't begin with "Re:", assuming
instead that you've written your own subject line, rather than having
a generated one.


BTW the string "Re: " is important, and should never be changed to any
other value unless you want to look like an MS-luser trying to break news
(e.g. such nonstandard prefixes break gnus-gather-threads-by-subject and
avoidance of duplicate "Re: " prefixes, amongst other things).