SICP 問題 3.10
(define w1 (make-withdraw 100)) ;; E2(balance:100)->E1(initial-amount:100)->global (w1 50) ;; E3->(amount:50)->E2(balance:50)->E1(initial-amount:100)->global (define (w2 (make-withdraw 100))) ;; E5(balance:100)->E4(initial-amount:100)->global