static Map<Int, OptimizedMultiSet<Int>> lcStream_rightPartnerMap(LCStreamingCompressor comp) {
  MultiSet<Int> ms = lcStream_itemCounts(comp);
  new Map<Int, OptimizedMultiSet<Int>> partners;
  for (int i = 0; i < comp.nItems(); i++) {
    IntPair p = comp.getPair(i);
    if (p != null) {
      int n = ms.get(i);
      //print("pair " + p + " => " + n);
      getOrCreate(() -> new OptimizedMultiSet, partners, p.a).add(p.b, n);
    }
  }
  
  for (IntPair p : overlappingIntPairs(comp.chain.toList()))
    getOrCreate(() -> new OptimizedMultiSet, partners, p.a).add(p.b);
  
  ret partners;
}

Began life as a copy of #1029110