per group value of gnus-summary-thread-gathering-function?

[Giorgos Keramidas]

Is it possible to customize `gnus-summary-thread-gathering-function' on
a per group basis?

I tried explicitly setting the thread gathering function in my ~/.gnus
file with:

  (setq gnus-summary-thread-gathering-function
        'gnus-gather-threads-by-references)

and the I added to the group parameters of a single group the item:

  (gnus-summary-thread-gathering-function . gnus-gather-threads-by-subject)

But this doesn't seem to work as I expected it would work.

The Gnus version I am using is hte bundled Gnus v5.13 snapshot from a
CVS Emacs snapshot checked out at 06-June-2008 (a bit old-ish but I
haven't been tracking Emacs very closely the last month or so).

What is the recommended way of getting different thread gathering logic
for some of the groups?