2.04¶

; Here is an alternative procedural representation of pairs. 
; For this representation, verify that (car (cons x y)) yields 
; x for any objects x and y.
(define (cons x y) 
    (lambda (m) (m x y)))
(define (car z)
    (z (lambda (p q) p)))
; What is the corresponding definition of cdr? (Hint: To verify 
; that this works, make use of the substitution model of Section 1.1.5.)

; Verify using substitution model
; 
; => (car (cons x y)) 
; => (car (lambda (m) (m x y)))
; => ((lambda (m) (m x y)) (lamda (p q)) p))
; => ((lambda (p q) p) x y)
; => x

; `cdr` can be defined as,
(define (cdr z)
    (z (lambda (p q) q)))