ack.m

    ack 0 n = n+1
    ack (m+1) 0 = ack m 1
    ack (m+1) (n+1) = ack m (ack (m+1) n)
    ack m n = error "ack applied to -ve or fractional arg"

This script defines Ackermann's function, beloved of recursion theorists. Example

        ack 3 3

yields 61, after a huge amount of recursion. Can only be called for small arguments, because the values get so big.