SICP Exercise 2.48 directed-line-segment

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.

 SOLUTION
The code and tests are here.

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