ack(0,0) = 1   ack(0,1) = 1   ack(0,2) = 1   ack(0,3) = 1
ack(1,0) = 2   ack(1,1) = 2   ack(1,2) = 2   ack(1,3) = 2
ack(2,0) = 4   ack(2,1) = 4   ack(2,2) = 4   ack(2,3) = 4
ack(3,0) = 5   ack(3,1) = 6   ack(3,2) = 8   ack(3,3) = 16