+(defun notmuch-search-filter-by-tag (tag)
+ "Filter the current search results based on a single tag.
+
+Runs a new search matching only messages that match both the
+current search results AND that are tagged with the given tag."
+ (interactive "sFilter by tag: ")
+ (notmuch-search (concat notmuch-search-query-string " and tag:" tag)))
+