Fibs challenge

program which will:
Display the largest fibs you can in your language of choice, but using only your own code
(no non-standard libraries like multiple precision arithmetic).


(define increment list)
#<unspecified>
(define decrement car)
#<unspecified>
(define zero '())
#<unspecified>
(define one (increment zero))
#<unspecified>
(define two (increment one))
#<unspecified>
(define is-zero? null?)
#<unspecified>
(define (is-one? n)
  (and (not (is-zero? n))
       (is-zero? (decrement n))))
#<unspecified>

(define (add num1 num2)
  (if (is-zero? num1)
    num2
    (add (decrement num1) (increment num2))))
#<unspecified>

(define (sub num1 num2)
  (if (is-zero? num2)
    num1
    (sub (decrement num1) (decrement num2))))
#<unspecified>

(define (fibs n)
  (cond ((is-zero? n) zero)
        ((is-one? n) one)
        (else (add (fibs (sub n one)) (fibs (sub n two))))))
#<unspecified>

(display (fibs '()))
()#<unspecified>
(display (fibs '(())))
(())#<unspecified>
(display (fibs '((()))))
(())#<unspecified>
(display (fibs '(((())))))
((()))#<unspecified>
(display (fibs '((((()))))))
(((())))#<unspecified>
(display (fibs '(((((())))))))
(((((())))))#<unspecified>
(display (fibs '((((((()))))))))
((((((((()))))))))#<unspecified>
(display (fibs '(((((((())))))))))
(((((((((((((())))))))))))))#<unspecified>
(display (fibs '((((((((()))))))))))
(((((((((((((((((((((())))))))))))))))))))))#<unspecified>
(display (fibs '(((((((((())))))))))))
((((((((((((((((((((((((((((((((((()))))))))))))))))))))))))))))))))))#<unspecified>