anchor.diff:          (reset-current-user))
       (log-in (create-user params))
       (flash "Welcome to the Sample App!")
  -    (redirect (format nil "/users/~A" (current-user-id))))
  +    (redirect-back-or (format nil "/users/~A" (current-user-id))))
     (redirect "/users/new"))
   
   (defroute "/users/:id" (&key id)
  
positive.code:   (gethash :user-id *session* nil))
  
  (defun reset-current-user ()
    (remhash :user-id *session*))
  
  (defun log-in (user)
    (reset-current-user)
  
negative.code: (defun logged-in-user ()
    (unless (logged-in-p)
      (progn
        (store-location)
        (flash "Please login.")
        (redirect "/login"))))
  
  