⇒(append '(a 1) (cons 'b (append '(2) '(c 3))))
⇒(append '(a 1) (cons 'b (cons 2 (append '() '(c 3)))))
⇒(append '(a 1) (cons 'b (cons 2 '(c 3))))
⇒(append '(a 1) (cons 'b '(2 c 3)))
⇒(append '(a 1) '(b 2 c 3))
⇒(cons 'a (append '(1) '(b 2 c 3)))
⇒(cons 'a (cons 1 (append '() '(b 2 c 3))))
⇒(cons 'a (cons 1 '(b 2 c 3)))
⇒(cons 'a '(1 b 2 c 3))
⇒'(a 1 b 2 c 3)
(define (append x z)
(cond ((null? x) z)
(else (cons (car x) (append (cdr x) z)))