nnmail-expiry-wait-function: broke, or how-to?

[mdb@cdc.noaa.gov (Mark Borges)]

After reading the info page, I thought I could set the variable
`nnmail-expiry-wait-function' to get finer-grain control of the expiry
process. At least, that's what the doc-string:

------------------------------------------------------------------------------
nnmail-expiry-wait-function's value is [byte-code elided]

Documentation:
*Variable that holds function to specify how old articles should be
before they are expired.
  The function will be called with the name of the group that the
expiry is to be performed in, and it should return an integer that
says how many days an article can be stored before it is considered
'old'. 

Eg.:

(setq nnmail-expiry-wait-function
      (lambda (newsgroup)
        (cond ((string-match "private" newsgroup) 31)
              ((string-match "junk" newsgroup) 1)
	      (t 7))))
------------------------------------------------------------------------------

leads me to believe. So I set it to this:

------------------------------------------------------------------------------
(setq nnmail-expiry-wait-function (lambda (group)
			       (cond
				((string= "nnml:lists.zsh" group)
				 30)
				((string= "nnml:lists.xemacs" group)
				 10)
				((string= "nnml:lists.fvwm" group)
				 10)
				((string= "nnml:lists.NCARgraphics" group)
				 10)
				((string= "nnml:lists.raiders" group)
				 4)
				(t
				 7))
			       ))
------------------------------------------------------------------------------

But articles in my nnml mail folders are not being expired, e.g., I
still have an (unticked) article dated 13 Nov in my lists.fvwm mail
group, well beyond the 10 days I want to keep it. (BTW, will ticked
articles be excluded from the expiry process?).

So either (a) I'm misunderstanding how this works, or (b) it's broke.
But it can't possibly be (b), so it must be (a).

I know this function should work, because when I'm in the *Group*
buffer and eval this function:

------------------------------------------------------------------------------
(defun my-expiry-wait-function (group)
			       (cond
				((string= "nnml:lists.zsh" group)
				 30)
				((string= "nnml:lists.xemacs" group)
				 10)
				((string= "nnml:lists.fvwm" group)
				 10)
				((string= "nnml:lists.NCARgraphics" group)
				 10)
				((string= "nnml:lists.raiders" group)
				 4)
				(t
				 7))
			       )
------------------------------------------------------------------------------

via
	ESC-ESC (my-expiry-wait-function (gnus-group-group-name))

the correct integer gets returned.

I see there is a variable called `gnus-newsgroup-auto-expire', whose
value is nil by default, but it's doc-string:

------------------------------------------------------------------------------
Documentation:
not documented as a variable.
------------------------------------------------------------------------------

makes me a bit leery of messing with it.

Must I put gnus-group-expire-articles() on a hook function? If so, any
suggestions on which one? Ideally, I'd like Gnus to assume auto-expiry
of the groups I've gone to the trouble of listing in
nnmail-expiry-wait-function, but I can do anything; I just need a bit
more info.

Thanks,

  -mb-