Exercise 2.83.  Suppose you are designing a generic arithmetic system 
for dealing with the tower of types shown in figure 2.25: integer, 
rational, real, complex. For each type (except complex), design a 
procedure that raises objects of that type one level in the tower. Show 
how to install a generic raise operation that will work for each type 
(except complex).

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(define (int->rat n) (make-rat n 1))
(define (rat->real rat) (/ (numer rat) (denom rat)))
(define (real->complex r) (make-from-real-imag r 0))

(put 'raise '(int) int->rat)
(put 'raise '(rat) rat->real)
(put 'raise '(real) real->complex)

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fixed in meeting:

(define (int->rat n) (make-rat (contents n) 1))
(define (rat->real rat) (* 1.0 (/ (numer rat) (denom rat))))
(define (real->complex r) (make-from-real-imag (contents r) 0))