Exercise 2.48.  A directed line segment in the plane can be represented 
as a pair of vectors -- the vector running from the origin to the 
start-point of the segment, and the vector running from the origin to 
the end-point of the segment. Use your vector representation from 
exercise 2.46 to define a representation for segments with a constructor 
make-segment and selectors start-segment and end-segment.

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I'm not using exercise 2.46 here so I seem to have missed the point of 
this question.

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