+ <b>thread:{<notmuch</b> <b>query>}</b>
+ If notmuch is built with <b>Xapian</b> <b>Field</b> <b>Processors</b> (see below),
+ threads may be searched for indirectly by providing an arbitrary
+ notmuch query in <b>{}</b>. For example, the following returns threads
+ containing a message from mallory and one (not necessarily the
+ same message) with Subject containing the word "crypto".
+
+ % notmuch search 'thread:"{from:mallory}" and thread:"{subject:crypto}"'
+
+ The performance of such queries can vary wildly. To understand
+ this, the user should think of the query <b>thread:{<something>}</b> as
+ expanding to all of the thread IDs which match <b><something></b>; not‐
+ much then performs a second search using the expanded query.
+