static <A, C extends Collection<A>> C assertContains(C c, A y) {
  ret assertContains(null, c, y);
}

static <A, C extends Collection<A>> C assertContains(S msg, C c, A y) {
  if (!contains(c, y))
    fail((msg != null ? msg + ": " : "") + y + " not contained in " + c);
  ret c;
}

sS assertContains(S a, S b) {
  if (!contains(a, b))
    fail(quote(b) + " not contained in " + quote(a));
  ret a;
}