Scheme implementation of natural numbers using BigNum representation

BigNum.rkt

#lang racket
; Zafer Cavdar
; Scheme implementation of natural numbers using BigNum representation

(define zero
  (lambda ()
    (cons 0 '())))

(define is-zero?
  (lambda (n)
    (eq? (car n) car((zero)))))

(define successor
  (lambda (n)
    (let ([x (+ (car n) 1)])
      (if (>= x 10)
          (if (null? (cdr n))
              (cons 0 (cons 1 '()))
              (cons 0 (successor (cdr n))))
          (cons x (cdr n))
          ))))

(define x (zero))

(define predecessor
  (lambda (n)
    (let ([x (- (car n) 1)])
      (if (< x 0)
          (if (null? (cdr n))
              'error-only-positive-numbers
              (cons 9 (predecessor (cdr n))))
          (cons x (cdr n))
          ))))

; Summation and substraction operations
(define increment
  (lambda (n t)
    (if (= t 0)
        n
        (increment (successor n) (- t 1)))))

(define decrement
  (lambda (n t)
    (if (= t 0)
        n
        (decrement (predecessor n) (- t 1)))))