code: void push_slot( int d, int n ) {
      assert d==0;                // Should use a fcn's closure for d>1
      int idx = _display[_tod-d]+n;
      push(1);
      _ary[_sp-1] = addRef(_ary[idx]);
      _d  [_sp-1] =        _d  [idx];
      _fcn[_sp-1] = addRef(_fcn[idx]);
      _str[_sp-1] =        _str[idx];
      assert _ary[0]==null || check_refcnt(_ary[0].anyVec());
    }
docs: Copy from display offset d, nth slot