forward is ackermann ( m n --> r )
  [ over 0 = iff
      [ nip 1 + ] done
    dup 0 = iff
      [ drop 1 - 1
        ackermann ] done
     over 1 - unrot 1 -
     ackermann ackermann ]   resolves ackermann ( m n --> r )

  3 10 ackermann echo