sclass LineComp_PairCounts {
  new MultiSet<Pair<Int>> counts;
  MultiSetMap<Int, Pair<Int>> byCount = multiSetMap_innerTreeSet_outerRevTreeMap();
  
  void addAll(Iterable<Pair<Int>> l) { fOr (Pair<Int> p : l) add(p); }
  void removeAll(Iterable<Pair<Int>> l) { fOr (Pair<Int> p : l) remove(p); }
  
  // increment pair's count
  void add(Pair<Int> p) {
    int count = counts.get(p);
    byCount.remove(count, p);
    counts.add(p);
    byCount.put(count+1, p);
  }
  
  // decrement pair's count
  void remove(Pair<Int> p) {
    int count = counts.get(p);
    if (count == 0) fail("Can't remove pair " + p);
    byCount.remove(count, p);
    counts.remove(p);
    if (count-- > 0) byCount.put(count, p);
  }
  
  Pair<Int> mostPopularDuplicate() {
    ret toInt(firstKey(byCount)) < 2 ? null : firstValue(byCount);
  }
  
  int numberOfDuplicates() {
    ret counts.size()-l(byCount.get(1));
  }
  
  int getCount(Pair<Int> p) {
    ret counts.get(p);
  }
}

Began life as a copy of #1028215