Show the current thread with a different filter (i.e., open messages
in the thread matching the new query).

Bound to 'l' for "limit".

Note that it is not the same as filter in search mode as it replaces
the existing query rather than ANDing with it (but it does keep the
thread-id part of the query).
---
 emacs/notmuch-show.el |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 30b26d1..4b4d663 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -42,6 +42,7 @@
 (declare-function notmuch-search-next-thread "notmuch" nil)
 (declare-function notmuch-search-show-thread "notmuch" nil)
 (declare-function notmuch-update-tags "notmuch" (current-tags tag-changes))
+(declare-function notmuch-read-query "notmuch" (prompt))
 
 (defcustom notmuch-message-headers '("Subject" "To" "Cc" "Date")
   "Headers that should be shown in a message, in this order.
@@ -1157,6 +1158,7 @@ reset based on the original query."
 	(define-key map "s" 'notmuch-search)
 	(define-key map "m" 'notmuch-mua-new-mail)
 	(define-key map "f" 'notmuch-show-forward-message)
+	(define-key map "l" 'notmuch-show-filter-thread)
 	(define-key map "r" 'notmuch-show-reply-sender)
 	(define-key map "R" 'notmuch-show-reply)
 	(define-key map "|" 'notmuch-show-pipe-message)
@@ -1399,6 +1401,16 @@ current thread."
   "Mark the current message as read."
   (notmuch-show-tag-message "-unread"))
 
+(defun notmuch-show-filter-thread ()
+  "Filter or LIMIT the current thread based on a new query string.
+
+Reshows the current thread with matches defined by the new query-string."
+  (interactive)
+  (setq notmuch-show-query-context (notmuch-read-query "Filter thread: "))
+  (when (string= notmuch-show-query-context "")
+    (setq notmuch-show-query-context nil))
+  (notmuch-show-refresh-view t))
+
 ;; Functions for getting attributes of several messages in the current
 ;; thread.
 
-- 
1.7.9.1