+;; The following variable can be overridden by let bindings.
+(defvar notmuch-show-insert-header-p-function 'notmuch-show-insert-header-p
+ "Specify which function decides which part headers get inserted.
+
+The function should take two parameters, PART and HIDE, and
+should return non-NIL if a header button should be inserted for
+this part.")
+