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:

  
    
      
        (
        
          x
          1
        
        ,
        
          y
          1
        
        )
        +
        (
        
          x
          2
        
        ,
        
          y
          2
        
        )
      
      
        =
      
      
        (
        
          x
          1
        
        +
        
          x
          2
        
        ,
        
          y
          1
        
        +
        
          y
          2
        
        )
        ,
      
    
    
      
        (
        
          x
          1
        
        ,
        
          y
          1
        
        )
        −
        (
        
          x
          2
        
        ,
        
          y
          2
        
        )
      
      
        =
      
      
        (
        
          x
          1
        
        −
        
          x
          2
        
        ,
        
          y
          1
        
        −
        
          y
          2
        
        )
        ,
      
    
    
      
        s
        ⋅
        (
        x
        ,
        y
        )
      
      
        =
      
      
        (
        s
        x
        ,
        s
        y
        )
        .