The Bactra Review   Permutation City
I can't resist a few technicalities here. To prevent paradoxes, the relation which generates causal chains has to be a partial ordering of the set of events, but the relation which generates spatio-temporal proximity needn't. Since all universes are assembled out of the same set of events, however, the ancestrals of both these relations must have domains whose cardinality is no larger than the cardinal number of the set of events. (It could be smaller; for instance, universes consisting of a single event.) All this would an exemplification of Russell's ``relation-arithmetic'' from the Principia.