static L<Int> lineComp_replacePairWithUpdatingPairCount(IContentsIndexedList2<Int> l, Int pa, Int pb, Int y, LineComp_PairCounts pairCounts) {
  int i = 0;
  while (true) {
    i = indexOfIntPairInContentsIndexedList(l, pa, pb, i);
    if (i < 0) break;
    
    int to = i+2, to_new = i+1;

    pairCounts.removeAll(overlappingIntPairs(subList(l, i-1, to+1)));
    l.set(i, y);
    l.remove(i+1);
    pairCounts.addAll(overlappingIntPairs(subList(l, i-1, to_new+1)));
    i = to_new;
  }
  ret l;
}

Began life as a copy of #1028216