static int lastIndexOf(S a, S b) {
  ret a == null || b == null ? -1 : a.lastIndexOf(b);
}

static int lastIndexOf(S a, char b) {
  ret a == null ? -1 : a.lastIndexOf(b);
}

// starts searching from i-1
static <A> int lastIndexOf(L<A> l, int i, A a) {
  if (l == null) ret -1;
  for (i = min(l(l), i)-1; i >= 0; i--)
    if (eq(l.get(i), a))
      ret i;
  ret -1;
}

static <A> int lastIndexOf(L<A> l, A a) {
  if (l == null) ret -1;
  for (int i = l(l)-1; i >= 0; i--)
    if (eq(l.get(i), a))
      ret i;
  ret -1;
}