ackermann: function [m,n][
    (m=0)? -> n+1 [
        (n=0)? -> ackermann m-1 1
               -> ackermann m-1 ackermann m n-1
    ]
]

loop 0..3 'a [
    loop 0..4 'b [
        print ["ackermann" a b "=>" ackermann a b]
    ]
]