I'll just add this to the record as well in case it is useful to someone
searching for examples of how to use this functionality:

(defun -notmuch-flag (format-string msg)
  (let ((tags (plist-get msg :tags)))
    (format format-string
            (cond
             ((member "deleted" tags) "D")
             ((member "unread" tags) "U")
             ((member "inbox" tags) "I")
             (t " ")))))


(setq notmuch-tree-result-format
      '(("date" . "%12s ")
        (-notmuch-flag . "%s ")
     (-notmuch-authors-or-to . "%-25s")
     ((("tree" . "%s")
       ("subject" . "%s"))
      . " %s")))

It's a single letter status flag to indicate what stage of processing an
email is at -- obviously targeted for the peculiarities of my personal
workflow but should be easy to adapt.
_______________________________________________
notmuch mailing list -- notmuch@notmuchmail.org
To unsubscribe send an email to notmuch-leave@notmuchmail.org