static L<Int> lineComp_replacePairWithUpdatingPairCount_v2(IContentsIndexedList2<Int> l, Int pa, Int pb, Int y, LineComp_PairCounts pairCounts) {
  int i = 0, copyFrom = 0, copyTo = 0, n = l(l);
  while (true) {
    i = indexOfIntPairInContentsIndexedList(l, pa, pb, i);
    if (i < 0) break;
    
    if (copyFrom != copyTo) {
      while (copyFrom < i)
        l.set(copyTo++, l.get(copyFrom++));
    } else
      copyTo = copyFrom = i;
      
    pairCounts.removeAll(overlappingIntPairs(subList(lineComp_listWithGap(l, copyTo, copyFrom), copyTo-1, copyTo+3)));
    l.set(i, y);
    copyFrom += 2;
    copyTo++;
    pairCounts.addAll(overlappingIntPairs(subList(lineComp_listWithGap(l, copyTo, copyFrom), copyTo-2, copyTo+1)));
    i += 2;
  }
  if (copyFrom != copyTo) {
    while (copyFrom < n)
      l.set(copyTo++, l.get(copyFrom++));
    truncateList(l, copyTo);
  }
  ret l;
}

Began life as a copy of #1028240