#
Representation of line segments on the plane.
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Consider the problem of representing line segments in a plane. Each segment is represented as a pair of points: a starting point and an ending point. Define a constructor make-segment and selectors start-segment and end-segment that define the representation of segments in terms of points. Furthermore, a point can be represented as a pair of numbers: the x coordinate and the y coordinate. Accordingly, specify a constructor make-point and selectors x-point and y-point that define this representation. Finally, using your selectors and constructors, define a procedure midpoint-segment that takes a line segment as argument and returns its midpoint (the point whose coordinates are the average of the coordinates of the endpoints). To try your procedures, you'll need a way to print points:

```
(define (print-point p)
(newline)
(display "(")
(display (x-point p))
(display ",")
(display (y-point p))
(display ")"))
```

```
(define p1 (make-point 2 3))
(define p2 (make-point 4 5))
(define s (make-segment p1 p2))
(check-equal? (make-segment p1 p2) '((2 . 3) 4 . 5))
(check-equal? (midpoint-segment s) '(3 . 4))
(check-equal? (start-segment s) p1)
(check-equal? (end-segment s) p2)
(check-equal? (x-point p1) 2)
(check-equal? (y-point p2) 5)
```