(defun type= (l)
	   (if (cdr l)
	       (if (eq (type-of (car l)) (type= (cdr l)))
		   (type-of (car l))
		   nil)
	       (type-of (car l))))
(deftype list-of (type)
	  `(and (cons ,type *)
		(satisfies type=)))

(typep '(a b c d) '(list-of symbol))
T
(typep '(a b 1 d) '(list-of symbol))
NIL