docs: function (cb, key, shiftKey, ctrlKey) {
      var effectiveShiftKey = shiftKey || false;
      var effectiveCtrlKey = ctrlKey || false;
      return function (event) {
        var keyMatched = typeof key === 'string' ?
          event.key === key :
          Util.find(key, function (k) { return k === event.key; });
  
        if (keyMatched && event.shiftKey === effectiveShiftKey && event.ctrlKey === effectiveCtrlKey) {
          cb(event);
        }
      };
    }