Add TODO

Author: mjackson

modules/useRoutes.js

@@ -216,6 +216,8 @@ function useRoutes(createHistory) {
      * gracefully handle errors and redirects.
      */
     function listen(listener) {
+      // TODO: Only use a single history listener. Otherwise we'll
+      // end up with multiple concurrent calls to match.
       return history.listen(function (location) {
         if (state.location === location) {
           listener(null, state)