An arm of an Occam ALT statement may include an input guard-a receive (?) operation-in which case the arm can be chosen only if a potential partner is trying to send a matching message. One could imagine allowing output guards as well: send (1) operations that would allow their arm to be chosen only if a potential partner were trying to receive a matching message. Neither Occam nor CSP (as originally defined) permits output guards. Can you guess why? Suppose you wished to provide them. How would the implementation work?