  We give a short, self-contained proof of Stanley's reciprocity theorem for a
rational cone K \subset R^d. Namely, let sigma_K (x) = sum_{m \in K \cap Z^d}
x^m. Then sigma_K (x) and sigma_int(K) (x) are rational functions which satisfy
the identity sigma_K (1/x) = (-1)^d sigma_int(K) (x). A corollary of Stanley's
theorem is the Ehrhart-Macdonald reciprocity theorem for the lattice-point
enumerator of rational polytopes. A distinguishing feature of our proof is that
it uses neither the shelling of a polyhedron nor the concept of finite additive
measures. The proof follows from elementary techniques in contour integration.
