 Although snapbacks are not possible in the basic variant (as it is necessary to make a ko threat before any consecutive capture occurs), they can be explicitly allowed with an extra rule while retaining the property that all known forced cycles are impossible.Environmental Go, also called Coupon Go, invented by Elwyn Berlekamp, adds an element of mathematical precision to the game by compelling players to make quantitative decisions.In lieu of playing a stone, a player may take the highest remaining card from a pack of cards valued in steps of ½ from ½ to 20