Let the objects consist of pairs, ( $A, A^{\prime}$ ), where $A$ is a set and $A^{\prime}$ is a subset of $A$. Let a morphism from $\left(A, A^{\prime}\right)$ to $\left(B, B^{\prime}\right)$ consist of a mapping $\varphi$ from set $A$ to set $B$ such that, whenever $a^{\prime}$ is in $A^{\prime}, \varphi\left(a^{\prime}\right)$ is in $B^{\prime}$. Let composition of morphisms be composition of mappings. Prove that this is a category. Discuss monomorphisms, epimorphisms, direct products, and direct sums.