code: function updateInkBarClassName () {
      var elements = getElements();
      var newIndex = ctrl.selectedIndex,
          oldIndex = ctrl.lastSelectedIndex,
          ink      = angular.element(elements.inkBar);
      if (!angular.isNumber(oldIndex)) return;
      ink
          .toggleClass('md-left', newIndex < oldIndex)
          .toggleClass('md-right', newIndex > oldIndex);
    }
docs: Adds left/right classes so that the ink bar will animate properly.