code: function incrementIndex (inc, focus) {
      var newIndex,
          key   = focus ? 'focusIndex' : 'selectedIndex',
          index = ctrl[ key ];
      for (newIndex = index + inc;
           ctrl.tabs[ newIndex ] && ctrl.tabs[ newIndex ].scope.disabled;
           newIndex += inc) { /* do nothing */ }
  
      newIndex = (index + inc + ctrl.tabs.length) % ctrl.tabs.length;
  
      if (ctrl.tabs[ newIndex ]) {
        ctrl[ key ] = newIndex;
      }
    }
docs: This moves the selected or focus index left or right. This is used by the keydown handler.
  @param {number} inc amount to increment
  @param {boolean} focus true to increment the focus index, false to increment the selected index