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