sub sort_stack {

  local $depth = pop @stack;
  local $sort_depth = $depth;

  for (1..$depth) {
    for (1..$sort_depth) {

      local $top  = pop @stack;
      local $next = pop @stack;

      if ($top gt $next) {

        $top  ^= $next;
        $next ^= $top;
        $top  ^= $next;

      }
      push @stack, $next, $top;
      rotate_up($sort_depth);
    }

    rotate_up($sort_depth);
    --$sort_depth;
  }
  push @stack, $depth;
}