static <A> int lCommonPrefix_lists_ext(L<A> a, L<A> b, int iB) {
  int i = 0, n = min(l(a), l(b)-iB);
  while (i < n && eq(a.get(i), b.get(iB+i)))
    ++i;
  ret i;
}

static <A> int lCommonPrefix_lists_ext(L<A> a, int iA, L<A> b, int iB) {
  int diff = iB-iA;
  int i = iA, n = min(l(a), l(b)-diff);
  while (i < n && eq(a.get(i), b.get(diff+i)))
    ++i;
  ret i;
}