sicp-ex-4.30

  
  
 ;; a 
 In begin expression, every expression will be evaluated using eval, and display is primitive function, it will call force-it to get x. 
  
 ;; b 
 original eval-sequence: 
 (p1 1) => (1 . 2) 
 (p2 1) => 1  . because (set! x (cons x '(2))) will be delayed, in function p, when evaluating it, it's a thunk. 
  
 Cy's eval-sequence: 
 (p1 1) => (1 . 2) 
 (p2 1) => (1 . 2). thunk (set! x (cons x '(2))) will be forced to evaluate. 
  
 ;; c 
 when using actual-value, it will call (force-it p), if p is a normal value, force-it will return p, just as never call actual-value 
  
 ;; d 
 I like Cy's method.