Exercise 2.4: 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 1.1.5.)