diff --git a/src/hb-set-private.hh b/src/hb-set-private.hh
index ccd4d8df5f74d38780321fc26d9cb7e5647a46d0..6841189ec654b7c5bcb60409f71af37b16b3b245 100644
--- a/src/hb-set-private.hh
+++ b/src/hb-set-private.hh
@@ -405,6 +405,7 @@ struct hb_set_t
     if (get_population () > larger_set->get_population ())
       return false;
 
+    /* TODO Optimize to use pages. */
     hb_codepoint_t c = INVALID;
     while (next (&c))
       if (!larger_set->has (c))