Breaking a really long thread

[David Mazieres]

For some reason, Google voice always includes a header of the form
"References: <+15555555555. at txt.google.com>" (where the 5s are my actual
phone number) when I get a text.  The result is that every SMS I've ever
received is in a single thread containing many thousands of messages.
And of course huge threads are annoyingly slow to open, particularly to
read a single 140-character message.

Is there any way to break an existing thread (so as to start over with a
smaller thread), or otherwise to tweak the threading rules so that a
particular References header gets ignored.  It's annoyingly slow to open
a thread with 10,000 messages just to read one SMS.  I'm almost tempted
to mangle the messages on delivery and remove the References header
before notmuch sees them, but it would be nice to have a cleaner
solution, as there are other situations in which one might want to
"reset" a really long thread.

Thanks,
David