parse arg n
If n < 1 Then Do
    n = 1
End

R = Ack(3,N)

say "Ack(3,"n"): "R
exit

Ack:
    PROCEDURE
    PARSE ARG M,N
    IF M<1 THEN RETURN N+1
    IF N<1 THEN RETURN Ack(M-1, 1)
    R1 = Ack(M, N-1)
    RETURN Ack(M-1, R1)