Threading without References header?

[Kai Grossjohann <grossjohann@charly.cs.uni-dortmund.de>]

There are some people who send me mail without any References header,
so Gnus can't really do threading right.

I use the following settings which at least sorts their articles
together with their "parents":

,-----
| (setq gnus-fetch-old-headers 1000)                           
| (setq gnus-thread-ignore-subject t)                          
| (setq gnus-summary-make-false-root 'empty)
`-----

I observe, though, that the fetch-old-headers mechanism doesn't work
for these references-less mails.  Ie I type 9 RET to enter a group and
if there is a references-less followup among these 9 articles, Gnus
doesn't fetch the parent.

Is there some magic that I can use to get what I want?  Or did I turn
of the feature with one of the variables above?

kai
-- 
Life is hard and then you die.