Exercise 2.2.  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 ")"))

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; segments

(define (make-segment start end) (cons start end))

(define (start-segment segment) (car segment))

(define (end-segment segment) (cdr segment))

; points

(define (make-point x y) (cons x y))

(define (x-point) (car point))

(define (y-point) (cdr point))

; midpoint

(define (midpoint-segment segment)
  (make-point (average (x-point (start-segment segment))
                       (x-point (end-segment segment)))
              (average (y-point (start-segment segment))
                       (y-point (end-segment segment)))))