notmuch.el and replying to multiple emails at once

[Matt Armstrong <matt@rfc20.org>]

A feature I miss from Gnus is being able to reply to multiple emails at
once.  Has anyone else found to be a missing feature?  I wonder if it
would be easy to add to notmuch's Emacs MUA.

Background: In Gnus' equivalent to notmuch.el's tree mode it is possible
to "mark" multiple messages at once in a manner similar to dired for
directory contents.  Gnus has a number of different operations that can
be performed on messaged so marked.  One is reply.  When replying to
multiple messages at once, the In-Reply-To: header is set appropriately,
and the quoted contents of each is right there in the message buffer.

I used to use this feature of Gnus frequently, especially in busy
threads.  I often find multiple messages that need replying to, but that
replying in one message tends to be more effective than scattered
replies to multiple messages.

Problem is, I can't think of an unobtrusive way to add this to notmuch
without adding a lot of machinery.