static <A> int smartIndexOfSubList(L<A> x, L<A> y) {
  ret smartIndexOfSubList(x, y, 0);
}

static <A> int smartIndexOfSubList(L<A> x, L<A> y, int i) {
  outer: for (; i+l(y) <= l(x); i++) {
    for (int j = 0; j < l(y); j++)
      if (neq(x.get(i+j), y.get(j)))
        continue outer;
    break;
  }
  ret i;
}

static <A> int smartIndexOfSubList(L<A> x, A[] y, int i) {
  outer: for (; i+l(y) <= l(x); i++) {
    for (int j = 0; j < l(y); j++)
      if (neq(x.get(i+j), y[j]))
        continue outer;
    break;
  }
  ret i;
}

Began life as a copy of #1002135