That's a bogus definition. A thread is defined by its subject line: this
is what you specify in your killfile when you want to plonk a thread.
The subject line is not affected by any such issues... Furthermore,
it provides a definition that is consistent on both Usenet and mailing
list discussions.
What exactly do you mean by "subject line"? Obviously the messages
with headers
Subject: foo
Subject: Re: foo
belong in the same "thread," according to your non-References-based
definition (which I don't agree with, even though Google does). How
about
Subject: RE: foo
Subject: Fwd: foo
Subject: [OT] Re: foo
Subject: Re: [OT] Re: foo
Subject: Re: [OT] foo
Subject: Re: fo...
Subject: bar, was Re: foo
Subject: [bar] Re: foo
I've ordered those subject lines roughly in the order I'd expect to
see a hypothetical Subject-threading-based newsreader accept them
as co-threaded with Subject: foo. IMO all of them would be accepted
as co-threaded by the average human reader.
And then Google accepts Subject headers like these:
Subject: foo
Subject: foo
Subject: FOO
Subject: Re: FOO
as co-threaded with Subject: foo, even though any human or
References-based reader would see immediately that capitalization is
significant in Subject headers, and no thread can have more than one
"root" message (indicated by the lack of Re: on the first three
example subject lines).
The major problem with Subject-based threading, of course, is that
you end up with "threads" like this one:
http://groups.google.com/[email protected]
This is *exactly* why the concept of the unique Message-ID header
was invented[1], and why References-based "tree" threading is
intrinsically better than Subject-based threading. YMOV, but I
really don't know why.
-Arthur
[1] - If it's not, well, it should be.