Exercise 2.79.  Define a generic equality predicate equ? that tests the 
equality of two numbers, and install it in the generic arithmetic 
package. This operation should work for ordinary numbers, rational 
numbers, and complex numbers.

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(define (complex-eq? x y)
  (and (= (real-part x) (real-part y))
       (= (imag-part x) (imag-part y))))

(define (rat-eq? x y)
  (and (= (numer x) (numer y)) -- I assume these are normalized
       (= (denom x) (denom y))))

(put 'equ? '(complex complex) complex-eq?)
(put 'equ? '(rational rational) rational-eq?)
(put 'equ? '(scheme-number scheme-number) =)