+``(thread (of (id 1234@invalid)))``
+ Match any message in the same thread as the one with Message-Id "1234@invalid"
+
+``(thread (matching (from bob@example.com) (to bob@example.com)))``
+ Match any (messages in) a thread containing a message from
+ "bob@example.com" and a (possibly distinct) message to "bob at
+ example.com")
+