A similar pattern is used by `gnus-article-sort-by-author', but I didn't
see an easy way to unify these without also passing around functions by
header type.

Also, I haven't used `defsubst' before, so I leave it up to you if it
makes sense to use that for the `gnus-article-sort-by-newsgroups' and
`gnus-article-sort-by-newsgroups', respectively.

Thanks!

-- 
Alex.