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 ")"))