(let (a b (c 0) (table (make-hash-table)))
  (setf (gethash 0 table) 0)
  (setf (gethash 1 table) 1)
  (block NIL
    (flet ((thunk (k v)
             (declare (ignore v))
             (case c
               (0 (setf a k))
               (1 (setf b k))
               (T (return NIL)))
             (incf c)))
      (declare (dynamic-extent #'thunk))
      (maphash #'thunk table))))