(eval-when (:compile-toplevel :load-toplevel :execute)
  (defmacro define-arb-and-non-arb (form)
    (let ((extension nil))
      (declare (special extension))
      (labels ((expand-extension (form)
		 (if (listp form)
		     (if (eql (first form) '?extension)
			 (if extension (second form) (third form))
			 (mapcar #'expand-extension form))
		     form))))
      (list 'progn
	    (let ((extension t))
	      (declare (special extension))
	      (expand-extension form)) ;  undefined function: EXPAND-EXTENSION
	    (expand-extension form)))))