- opportunity cost.
Of these, the first is traditional in CGT; while the second accords much better with normal go-playing thinking. That's because tenuki is a basic strategic concept in go.
Elwyn Berlekamp has been proposing to use models of the second kind, with a stack of cards (tokens of definite numerical value) by the board. At every turn a player has the choice: play on the goban or take a card. A stack of cards properly tailored to this purpose is called an environment. Environmental go is a viable form of the game, with interest for the way it may show up decision-making processes.