docs: void push( int slots ) {
      assert 0 <= slots && slots < 1000;
      int len = _d.length;
      _sp += slots;
      while( _sp > len ) {
        _key= Arrays.copyOf(_key,len<<1);
        _ary= Arrays.copyOf(_ary,len<<1);
        _d  = Arrays.copyOf(_d  ,len<<1);
        _fcn= Arrays.copyOf(_fcn,len<<=1);
        _str= Arrays.copyOf(_str,len<<1);
      }
    }