'''From here on it's just an engineering problem to construct an arbitrarily large finite (and very slow!) computer. Our engineer has been given the tools--let him finish the job! [sic]'''

[After describing how to construct AND, OR, and NOT from glider collisions]

Elwyn R. Berlekamp, John H. Conway, and Richard K. Guy. Winning Ways for Your Mathematical Plays. Volume 4, Second edition. p. 949