It’s messy to have to relate; it’s neat to have once related. Just ask Jerry Seinfeld and his TV ex-girlfriend, Elaine Benes (Julia Louis-Dreyfus), who get to joke about sex on Seinfeld without having to actually have sex anymore. Ask Juliette Lewis and the ex-love-of-her-life, Brad Pitt, with whom she is now amicably promoting their August film, Kalifornia. Ask Woody Allen, who cast friendly ex-flame Diane Keaton in the role vacated by unfriendly ex-flame Mia Farrow in his next film, Manhattan Murder Mystery. As Allen might say in this case, the part wants what it wants.