docs: void tos_into_slot( int idx, String id ) {
      subRef(_ary[idx], _key[idx]);
      subRef(_fcn[idx]);
      Frame fr =                   _ary[_sp-1];
      _ary[idx] = fr==null ? null : addRef(new Frame(fr));
      _d  [idx] =                  _d  [_sp-1] ;
      _fcn[idx] =           addRef(_fcn[_sp-1]);
      _str[idx] =                  _str[_sp-1] ;
      _key[idx] = fr!=null ? id : null;
      assert _ary[0]== null || check_refcnt(_ary[0].anyVec());
    }