Python implementation of Tak.

tarai.py

#!/usr/bin/env python

def tarai(x, y, z):
    if x <= y:
        return y
    else:
        return tarai(
                tarai(x-1, y, z),
                tarai(y-1, z, x),
                tarai(z-1, x, y))

x = 12
y = 6
z = 0

print(tarai(x, y, z))