static <A> int lCommonPrefix_lists_comparator(L<A> a, L<A> b, Comparator<A> comparator) {
  int i = 0, n = min(l(a), l(b));
  while (i < n && eqWithComparator(comparator, a.get(i), b.get(i)))
    ++i;
  ret i;
}