Data migration with conjunctive queries

[epatters]

Implement data migration from C-sets to D-sets specified by a functor D -> Conj(C). Here Conj(C) is the category of conjunctive queries on C, which we take to be Diag^{op}(C), the category of diagrams in C regarded as limit diagrams. From this perspective, pullback data migration is the special case where the shape of each diagram is the terminal category {*}.