:ack
    %n !n %m !m

    $m 0 ==
    ( [$n 1 +]
      [$n 0 ==
        ( [$m 1 - 1 ack]
          [$m 1 - $m $n 1 - ack ack]
        ) if
      ]
    ) if
;

3 6 ack print nl
msec print

" " input