static L<Int> lineComp_replacePairWithUpdatingPairCount(IContentsIndexedList2<Int> l, IntPair p, L<Int> y, LineComp_PairCounts pairCounts) {
  int i = 0;
  Int pa = p.a, pb = p.b; // box them once
  while (true) {
    i = indexOfIntPairInContentsIndexedList(l, pa, pb, i);
    if (i < 0) break;
    
    int to = i+l(x), to_new = i+l(y);
    ifdef lineComp_replaceSublistWithUpdatingPairCount_debug
    print("Replacing " + subList(l, i, to) + " at " + i);
    endifdef
    
    pairCounts.removeAll(overlappingIntPairs(subList(l, i-1, to+1)));
    replaceSublist(l, i, to, y);
    pairCounts.addAll(overlappingIntPairs(subList(l, i-1, to_new+1)));
    i = to_new;
  }
  ret l;
}