>>> Good email software lets one change the subject in a reply, but retain
>>> the in-reply-to header, whose value is a message ID.
>>
>> Presumably most email clients get this right nowadays
>
> That's great. So maybe we can try to avoid a forest of threads by
> making more use of the inherent tree-like structure (and by choosing
> good subjects along the way).

Indeed, most email clients ought to cause deeply nested trees to be
created by default if people simply click "Reply" to the latest message.
Hence such trees are probably unavoidable over the long term with a
large pool of people participating.

Arthur can probably get the SRFI email archives in mbox or maildir
format. It should be reasonably easy to write a Scheme script to turn
them into HTML pages with a Discourse-forum-type presentation (i.e. show
a flat list of the messages in each thread sorted by date, but add links
that jump back to the parent message when the parent is further up). Or
any other presentation that is interesting.

> PS1 Maybe you know how to do it: Sometimes I want to reply to a
> message I can read on the list server but which I do not have
> (anymore) stored as an email. Do you know a simple way to emulate a
> reply-to?

I've been wanting the same functionality. No idea if it's possible.