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.