(defun summe (x)
(cond ((Null x) 0)
(( null (cdr x)) (car x) )
(( eq + (car(cdr x))) (summe (plussum x)) )
( T (summe (minussum x)))  ))

(defun plussum (x)
(cons (+ (car x)  (car (cdr(cdr x))) )  (cdr(cdr(cdr x )))   ) )

(defun minussum (x)
(cons (- (car x)  (car (cdr(cdr x))) )  (cdr(cdr(cdr x )))   ) )


(defun p (X) (r X X) )

(defun r (X Y) (a X nil Y))

(defun a (X Y Z)
(cond ((null X) (eq Y Z))
(T (a (cdr X) (cons (car X) Y) Z))))

(defun equal2 (x y)
(cond ((null x) (null y))
((eq (car x) (car y)) (equal2 (cdr x) (cdr y) )))