var previous = null, lastPrevious = null, memos = {};
      $rootScope.$on("$transitionStart", function(evt, $transition$) {
        lastPrevious = previous;
        previous = $transition$.from;

        $transition$.promise.then(commit).catch(revert);
        function commit() { lastPrevious = null; }
        function revert() { previous = lastPrevious; }
      });