Exercise 2.81: Louis Reasoner has noticed that
apply-generic may try to coerce the arguments to each other’s type even
if they already have the same type.  Therefore, he reasons, we need to put
procedures in the coercion table to 
coerce arguments of each type to
their own type.  For example, in addition to the
scheme-number->complex coercion shown above, he would do:


(define (scheme-number->scheme-number n) n)
(define (complex->complex z) z)

(put-coercion 'scheme-number 'scheme-number
              scheme-number->scheme-number)

(put-coercion 'complex 'complex 
              complex->complex)


 With Louis’s coercion procedures installed, what happens if
apply-generic is called with two arguments of type scheme-number
or two arguments of type complex for an operation that is not found in
the table for those types?  For example, assume that we’ve defined a generic
exponentiation operation:


(define (exp x y) 
  (apply-generic 'exp x y))

and have put a procedure for exponentiation in the Scheme-number
package but not in any other package:


;; following added to Scheme-number package
(put 'exp 
     '(scheme-number scheme-number)
     (lambda (x y) 
       (tag (expt x y)))) 
       ; using primitive expt


What happens if we call exp with two complex numbers as arguments?

 Is Louis correct that something had to be done about coercion with arguments of
the same type, or does apply-generic work correctly as is?

 Modify apply-generic so that it doesn’t try coercion if the two
arguments have the same type.