Quoth Andrei POPESCU on Apr 20 at 12:04 am:
> On Sb, 19 apr 14, 18:52:02, Eric wrote:
> > 
> > This may not actually be any help, but both hypermail and mhonarc agree
> > that two messages form a separate thread from the rest. I believe that
> > the latter, at least, is the JWZ algorithm.
> 
> mutt concurs.

Can anyone explain why JWZ *doesn't* have the same problem?  I don't
see how this heuristic doesn't doom it to the same fate:

  The References field is populated from the ``References'' and/or
  ``In-Reply-To'' headers. If both headers exist, take the first thing
  in the In-Reply-To header that looks like a Message-ID, and append
  it to the References header.

Given this, even considering only messages 18 and 52 (which "should"
be in different threads), JWZ should find the common "parent"
e.fraga@ucl.ac.uk and link them in to the same thread:

Add 18 (step 1)
- The combined "references" list is <ID17> <e.fraga@ucl.ac.uk>
- Creates and links containers 17 <- e.fraga@ucl.ac.uk <- 18 where the
  first two are empty

Add 52 (step 1)
- The combined "references" list is <ID31> <ID32> <ID39>
  <e.fraga@ucl.ac.uk>
- Creates and links containers 31 <- 32 <- 39
- Also considers container e.fraga@ucl.ac.uk, but this is already
  linked, so it doesn't change it
- Creates container 52 and links e.fraga@ucl.ac.uk <- 52 (step 1C)

18 and 52 will later get promoted over their empty parent (step 4),
but will remain in the same thread.

What am I missing?  Or are these other MUAs not using pure JWZ?