sS lcStream_rightPartnerMap(LCStreamingCompressor comp) {
  MultiSet<Int> ms = lcStream_itemCounts(comp);
  Map<Int, OptimizedMultiSet<Int>> partners = autoMap(OptimizedMultiSet);
  for (int i = 0; i < comp.nItems(); i++) {
    IntPair p = comp.getPair(i);
    if (p != null)
      partners.get(p.a).add(p.b, ms.get(i));
  }
  
  ret partners;
}