 There are many generalisations of this result, some of them involving the following notion. Let \(Z\) be a binary relation which relates states of \(A\) to states of \(B\). Then we call \(Z\) a bisimulation between \(A\) and \(B\) if Duplicator can use \(Z\) as a nondeterministic winning strategy in the back-and-forth game between \(A\) and \(B\) where the first pair of moves of the two players is to choose their starting states. In computer science the notion of a bisimulation is crucial for the understanding of \(A\) and \(B\) as systems; it expresses that the two systems interact with their environment in the same way as each other, step for step. But a little before the computer scientists introduced the notion, essentially the same concept appeared in Johan van Benthem’s PhD thesis on the semantics of modal logic (1976).