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 section 1.1.5.)

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(define (cdr z)
  (z (lambda (p q) q)))

This cons returns a function of one argument which returns the application 
of that argument to x and y.

By substitution:

  (car (cons x y))
→ (car (lambda (m) (m x y)))
→ ((lambda (m) (m x y)) (lambda (p q) p))
→ ((lambda (p q) p) x y)
→ x