static <A> LL<A> allPermutations(L<A> l) {
  new LL<A> out;
  allPermutations_impl(cloneList(l), l(l), out);
  ret out;
}

static <A> void allPermutations_impl(L<A> l, int n, LL<A> out) {
  if (n <= 1) {
    ping();
    out.add(cloneList(l));
  } else for(int i = 0; i < n; i++) {
    allPermutations_impl(l, n-1, out);
    swapElements(l, n % 2 == 0 ? i : 0, n-1);
  }
}