if (gp instanceof ALPacker) packer = (ALPacker) gp;
  Filler(Sizer sz, GenericPlugin gp, AggregationClosure ac,         AllocationResultDistributor ard,         PreferenceAggregator pa) {    _sz = sz;    _gp = gp;    _ac = ac;    _ard = ard;    _pa = pa;  }