Quasistrategy

A quasistrategy assigns a nonempty set of allowed moves to every position at which its player moves. It is winning when every play consistent with those allowed moves is won by that player.

An infinite game of perfect information is quasidetermined when one of the players has a winning quasistrategy. Choosing one allowed move at every relevant position refines a winning quasistrategy to a winning strategy in an infinite game whenever the required restricted choice principle holds.

For every move set $M$, the statement that every quasidetermined subset of $M^{\omega }$ is determined is equivalent in ZF to ${\mathsf{AC}}_{M^{<\omega }}(M)$. The forward direction encodes an arbitrary family of nonempty subsets of $M$ into a quasidetermined game; the reverse direction chooses one move from each value of a winning quasistrategy.