1: multi sub quicksort ( ) { () }
2: multi sub quicksort ( *$x, *@xs ) {
3:     my @pre  = @xs.grep{ $_ < $x };
4:     my @post = @xs.grep{ $_ >= $x };
5:     (@pre.quicksort, $x, @post.quicksort);
6: }
7: (1, 5, 2, 4, 3).quicksort.say;