followups go to expired group

[David Aspinwall <aspinwall@TimesTen.com>]

I use
  (setq nnmail-expiry-target "nnml:expired")
so that expired messages go to a 'expired' group.  I also use
  (: nnmail-split-fancy-with-parent)
in nnmail-split-fancy, so that followup messages go to the same group
as their parents.  The unfortunate effect of this is that if someone
replies to a message which has already expired, the reply goes to the
'expired' group.  I pretty much never look at that group, so I usually
don't even notice the new message.

Is there any way to get (: nnmail-split-fancy-with-parent) to work
except when the parent has been expired?  I guess I could move it from
the '(|' section of nnmail-split-fancy to the '(&' section so it gets
cross-posted, but I don't normally want that.