Exercise 2.84.  Using the raise  operation of exercise 2.83, modify the 
apply-generic procedure so that it coerces its arguments to have the 
same type by the method of successive raising, as discussed in this 
section. You will need to devise a way to test which of two types is 
higher in the tower. Do this in a manner that is ``compatible'' with the 
rest of the system and will not lead to problems in adding new levels to 
the tower.

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If possible, raise a to the type of b and return the raised a, or false.

(define (raise-to-type-of a b)
  (cond ((eq? (type-tag a) (type-tag b)) a)
        ((get 'raise (type-tag a)) (raise-to-type-of ((get 'raise (type-tag a)) a) b))
        (else false)))

(define (apply-generic op . args)
  (let ((type-tags (map type-tag args)))
    (let ((proc (get op type-tags)))
      (if proc
          (apply proc (map contents args))
          (if (= (length args) 2)
              (let ((raised-a (raise-to-type-of a b)))
                (if (raised-a)
                    (apply-generic raised-a b)
                    (let ((raised-b (raise-to-type-of b a)))
                      (if (raised-b)
                          (apply-generic a raised-b)
                          (error "No method for these types"))))))))))