If V is a linear space and S is a proper subset of V, and we
define a relation on V via v1 ~ v2 iff v1 - v2 are in S, a subspace
of V. We are given ~ is an equivalence relation, show that the set
of equivalence classes, V/S, is a vector space as well, where the
typical element of V/S is v + s, where v is any element of V.