sub ret_iter4 (@loops is copy) returns Ref {
    my $last = [*] @loops.map:{ $_.elems };
    my $iter = -1;
    return sub {
        my $i = ++$iter;
        return () if $iter >= $last;
        my $possible = $last;
        return @loops.map:{
            $possible /= $_.elems;
            my $this_iter_i = $i / $possible;
            $i %= $possible;
            $_[$this_iter_i];
        };
    };
}