# Exercise 2.4

We use the substitution model to verify that `(car (cons x y))`

yields `x`

.

```
(car (cons x y))
(car (lambda (m) (m x y))) ;using the definition of cons in the exercise
((lambda (m) (m x y)) (lambda (p q) p)) ;using the definition of car in this exercise
((lambda (p q) p) x y)
x
```

Using this procedural representation of pairs we can define `cdr`

as below.