One way to categorify the non-negative integers is to consider the category FinSet whose objects are finite sets and whose morphisms are functions. The isomorphism classes of objects in FinSet can be labeled "sets of cardinality 0, sets of cardinality 1," and so forth, so are a natural way of talking about the non-negative integers.

What I want is a good definition of a category FinSSet (where SSet stands for "super set") whose isomorphism classes can be thought of as "sets of cardinality k" for all integers k in a natural way. A natural candidate for the set of objects is the set of "Z_{2}-graded sets," i.e. pairs of sets (S_{0}, S_{1}). What we want is to define the cardinality of such a set as card(S_{0}) - card(S_{1}), so we want the isomorphism classes of objects to only depend on this number.

Unfortunately, I can't find a definition of the morphisms that actually accomplishes this. What should it be? For what it's worth, I've read "From Finite Sets to Feynman Diagrams" and I think John Baez gives up a little too early on the integers.