Puzzle: Why do some follow-ups thread and some not

[Steinar Bang <sb@metis.no>]

Emacs 19.31, Gnus 5.2.14 (but this behaviour I've observed since 5.1)

What makes some articles in mail folders thread, and some not?

Articles made with a follow-up with Gnus seem to thread, articles
created with a reply in RMAIL seem to thread.  Articles made with a
reply with Mew seem to thread.

Articles made with a reply with mh-e doesn't thread.  Ditto for exmh.
I've created a replcomps file that gives me In-reply-to headers and
References headers.

At the start, I *think* I got these to thread, but then they suddenly
stopped threading.

But all of this may be caused by erronous headers.  So here comes the
mysterious part:
 I have two articles.  One made by RMAIL (the one from Trond
 Kristiansen), one made by mh-e (the one from me).

I've tried hand editing the one from me, to see if I could get it to
thread, and I've been unsuccessful in doing so.

I've done the testing by editing, then starting a new emacs, then
killing all Gnus buffers and exiting emacs, answering "yes" on the
"unsaved buffers exits, do you really...etc.".  Then editing, and
starting a new emacs, answering "n" on the offer to use auto save
file (even when I've deleted "#.newsrc-dribble" it displays this
prompt). 

So here are the two headers.  As near as I can see, all headers that
should be relevant for threading are identical.  But one thread and
the other doesn't.

This isn't like, important.  But it really *bugs* me not knowing *why*
this happens...:-(

Is the information about the threading cached in some obscure place?
I've tried stripped everything relating to this mailing list out of
.newsrc.eld (without finding anything, when I refused the offer to
save). 



Header 1

Replied: Fri, 14 Jun 1996 11:33:20 +0200
Replied: "Trond Kristiansen <tk@metis.no> dk,jh,lro,ht"
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Date: Thu, 13 Jun 1996 12:30:29 +0200
Message-Id: <199606131030.MAA01099@client.metis.no>
From: Trond Kristiansen <tk@metis.no>
To: dk@metis.no
CC: jh@metis.no, lro@metis.no, ht@metis.no, sb@metis.no, lro@metis.no
In-reply-to: <31BFDACB.2698@metis.no> (message from Dag Karlsen on Thu, 13 Jun
	1996 11:09:31 +0200)
References: <31BFDACB.2698@metis.no>
Subject: Re: New 1.x API


Header 2

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Date: Fri, 14 Jun 1996 11:16:25 +0200
Message-Id: <199606140916.LAA20466@client.metis.no>
From: Steinar Bang <sb@metis.no>
To: Dag Karlsen <dk@metis.no>
CC: jh@metis.no, lro@metis.no, ht@metis.no, sb@metis.no, lro@metis.no
In-reply-to: <31BFDACB.2698@metis.no> (message from Dag Karlsen on Thu, 13 Jun
	1996 11:09:31 +0200)
References: <31BFDACB.2698@metis.no>
Subject: Re: New 1.x API