Exercise 2.46.  A two-dimensional vector v running from the origin to a 
point can be represented as a pair consisting of an x-coordinate and a 
y-coordinate. Implement a data abstraction for vectors by giving a 
constructor make-vect and corresponding selectors xcor-vect and 
ycor-vect. In terms of your selectors and constructor, implement 
procedures add-vect, sub-vect, and scale-vect that perform the 
operations vector addition, vector subtraction, and multiplying a vector 
by a scalar.

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(define make-vect cons)
(define xcor-vect car)
(define ycor-vect cdr)

(define (add-vect a b)
  (make-vect (+ (xcor-vect a) (xcor-vect b))
             (+ (ycor-vect a) (ycor-vect b))))

(define (sub-vect a b)
  (make-vect (- (xcor-vect a) (xcor-vect b))
             (- (ycor-vect a) (ycor-vect b))))

(define (scale-vect a n)
  (make-vect (* (xcor-vect a) n)
             (* (ycor-vect a) n)))