Ask HN: Idea for improving email threading

I believe we can all agree that grouping email by thread (à la Gmail, apple mail, etc.) is a great usability improvement; however, I find that all too often a single thread becomes very long, and (due to human laziness) the topic of discussion veers greatly from the subject line. I believe all threaded email readers should implement a command that 'breaks' a thread at any desired point within the thread, so that the emails prior to your 'break' command fall back into their original timeline order and your current topic of discussion can be separated from its full history. Thoughts?