(option '("x-accel-redirect") #f #t
                (lambda (opt name arg result)
                  (alist-cons 'x-accel-redirect?
                              (cond
                               ((member arg '(#f "yes" "true")) #t)
                               ((member arg '("false" "no") #f))
                               (else
                                (simple-format
                                 (current-error-port)
                                 "~A: invalid --x-accel-redirect value\n" arg)
                                (exit 1)))
                              (alist-delete 'x-accel-redirect? result))))))