RosettaCodeData/Task/Ackermann-function/Bracmat/ackermann-function-1.bracmat

( Ack
=   m n
  .   !arg:(?m,?n)
    & ( !m:0&!n+1
      | !n:0&Ack$(!m+-1,1)
      | Ack$(!m+-1,Ack$(!m,!n+-1))
      )
);