code: function clearModifier(event){
      var key = event.keyCode, k;
      if(key == 93 || key == 224) key = 91;
      if(key in _mods) {
        _mods[key] = false;
        for(k in _MODIFIERS) if(_MODIFIERS[k] == key) assignKey[k] = false;
      }
    }
docs: unset modifier keys on keyup