// a = old list, b = new list
static IntRange detectListInsertion(L a, L b) {
  int la = l(a), lb = l(b);
  if (la > lb) null;
  int i = 0;
  while (i < la && eq(a.get(i), b.get(i))) ++i;
  int j = i+lb-la;
  if (!subListsEqual(a, i, b, j, la-i)) null;
  ret intRange(i, j);
}