# SICP - Solution: Exercise 2.7

## Jan 9, 2019 07:09 · 59 words · 1 minute read

**Exercise 2.7:** Alyssa’s program is incomplete because she has not specified the implementation of the interval abstraction. Here is a definition of the interval constructor:

```
(define (make-interval a b) (cons a b))
```

Define selectors upper-bound and lower-bound to complete the implementation.

**Solution**

```
(define (make-interval a b) (cons a b))
(define (lower-bound x) (car x))
(define (upper-bound x) (cdr x))
```