I got the patch working and tested it today.  It worked fine for me.
It does something only when message-hierarchical-lists is defined, on
replying to a message.  Thanks again to Paul and Kai.

I am attaching the patch again; it worked against this morning's CVS
Gnus.  It being the first patch I've done for Gnus, I don't know if it
should go to the list or to someone in particular.

Paul Jarc wrote:
> Ted Zlatanov wrote:
>> Also, the message-hierarchical-lists should probably be generalized to
>> regular expressions for both the primary and secondary lists
>> (message-hierarchical-lists-regexps?).  I don't need this personally,
>> but perhaps someone else will find it useful.
>
> That would complicate the code; the two uses of assoc would have to be
> replaced by explicit iteration and regexp comparison.  We could wait
> for someone to ask for it, if it really is ever useful.

You're right, no one has been clamoring for this feature as it is.

Thanks
Ted