Can you edebug the function and see if it behaves the way it should
(evaluating the setq expression)?

>> I'm not familiar with emacs internals, so can't say where the problem
>> now: the bug is still here, or my testing strategy is wrong.
>>
>
> I think your test is alright in this case, and that a bug (maybe another
> one) remains.
>
>> P.S. (Offtopic): I'm also novice at maillists, so confused what should
>> I do now in my email client: Reply, Reply to All or Reply to
>> List. Sorry please if I did it wrong.
>
> The key is to keep commenters in the CC so they get your follow up.  How
> to do that varies between mail clients, but it sounds like Reply to All
> would be the best fit.
>
>
> Cheers,
>
> Eshel