HTTP: trait

  includes
    IIIR,
    Rational

  introduces
    address: Request -> URI
    penalty: Request, Entity -> Q
    date: Request -> Time

    get: Request -> Response

    object: Response -> Entity
    date: Response -> Time
    % could generalize this...
    vary: Response -> Bool

  asserts
    forall req: Request, e: Entity
     object(get(req)) \in represent(address(req), date(req));

     e \in represent(address(req), date(req)) =>
       penalty(req, object(get(req))) <= penalty(req, e);

     vary(get(req)) = false =>
       represent(address(req), date(req)) = { object(get(req)) }