static <A> L<O[]> allPermutations_arrays(L<A> l) {
  int n = factorial(l(l));
  L<O[]> out = new L(n);
  allPermutations_arrays_impl(cloneList(l), l(l), out);
  ret out;
}

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

Began life as a copy of #1010320