Emulate returns a sequence of shadow subjects created by applying the events in the history of the given pair to the shadow of its subject.

Evaluate returns a sequence of all possible subjects that can be obtained by executing the given initialisation function and then applying events to the subject.

NewPair creates a new pair.

History is a history of events.

Pair is a pair of a solution and a history.

Subject is an interface representing an entity that can walk through a history.