(defmacro write-with-rw-lock ((lock) &body body)
  (let ((locked (gensym)))
    `(let ((,locked nil))
       (unwind-protect
           (when (begin-write ,lock)
             (prog2
               (setf ,locked t)
               (locally ,@body)
               (when (end-write ,lock)
                 (setf ,locked nil))))
         (when ,locked
           (end-write ,lock))))))