Oh, ok. Didn't realize. I did the change a long time ago in my private
installation.

One difference in my version is the visibility of the filter in the
mode-line though. Doing C-u < multiple times adds every removed
category to the display in the mode-line.

I updated to 8.0.3 recently and did some changes to "my" version to
correspond to the evolution in the main repository. Here comes an
updated version:

(defun org-agenda-filter-by-category (strip)
  "Keep only those lines in the agenda buffer that have a specific category.
The category is that of the current line."
  (interactive "P")
  (if (and org-agenda-filtered-by-category
  org-agenda-category-filter)
      (org-agenda-filter-show-all-cat)
    (let ((cat (org-no-properties (get-text-property (point) 'org-category))))
      (cond
       ((and cat strip)
        (org-agenda-filter-apply
         (push (concat "-" cat) org-agenda-category-filter) 'category))
       ((and cat)
        (org-agenda-filter-apply
         (setq org-agenda-category-filter
      (list (concat "+" cat))) 'category))
       ((error "No category at point"))))))

I also attached a diff between the above and the code in 8.0.3.

Regards Gustav

On Thu, Jun 27, 2013 at 4:48 PM, Bastien <bzg@gnu.org> wrote:
>
> Hi Gustav,
>
> (please use plain text when posting code, other the code snippet
> is often mangled and not readable... thanks!)
>
> Gustav Wikström <gustav.erik@gmail.com> writes:
>
> > I'd like to propose an addition to the function
> > "org-agenda-filter-by-category"; that is, the function called when pushing
> > "<" in an agenda.
> >
> > As some might know adding a C-u in front of the command filters away the
> > current category instead of only listing it solemnly. I've made a change in
> > my source to allow multiple categories to be filtered away, instead of the
> > current functionality where only one category could be removed at a time.
>
> Using C-u < multiple times to exclude entries from multiple categories
> works here.  What version of Org are you using (M-x org-version RET)?
>
> Thanks,
>
> --
>  Bastien