ackermann = { m, n |
	when { m == 0 } { n + 1 }
		{ m > 0 && n == 0 } { ackermann(m - 1, 1) }
		{ m > 0 && n > 0 } { ackermann(m - 1, ackermann(m, n - 1)) }
}

p ackermann 3, 4  #Prints 125