static <A> IterableIterator<L<A>> allPermutations_iterator(L<A> theList) {
  ret iteratorFromFunction(new F0<A> {
    L<A> list = cloneList(theList);
    int[] stack = new int[l(theList)+1];
    int n = l(theList);
    
    A get() {
      findNext();
      if (n <= 1) {
        ++n; // return from subroutine
        ret cloneList(list);
      } else if (stack[n] < n)
        ++stack[n];
      }
      L<A> l = cloneList(list);
      
      if (stackPtr[n] < n)
        ++stackPtr[n];
      ret 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);
  }
}

Began life as a copy of #1010320