static <Int> L<Int> lineComp_replaceSublistWithUpdatingPairCount(L<Int> l, L<Int> x, L<Int> y, LineComp_PairCounts pairCounts) {
  if (x == null) ret l;
  
  int i = 0;
  while (true) {
    i = indexOfSubList(l, x, i);
    if (i < 0) break;
    
    int to = i+l(x), to_new = i+l(y);
    pairCounts.removeAll(subList(l, i-1, to+1));
    replaceSublist(l, i, to, y);
    pairCounts.addAll(subList(l, i-1, to_new+1));
    i = to_new;
  }
  ret l;
}