excessive thread fusing

[Austin Clements]

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 at ucl.ac.uk and link them in to the same thread:

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

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