static bool startsWith(S a, S b) {
  ret a != null && a.startsWith(b);
}

static bool startsWith(L a, L b) {
  if (a == null || l(b) > l(a)) ret false;
  for (int i = 0; i < l(b); i++)
    if (neq(a.get(i), b.get(i)))
      ret false;
  ret true;
}