Numbers from "The Little Schemer"

numbers.scm

(define (add1 x) (+ x 1))

(define (sub1 x) (- x 1))

(define (o+ x y)
  (cond ((= y 0) x)
        (else (o+ (add1 x) (sub1 y)))))

(define (o- x y)
  (cond ((= y 0) x)
        (else (o- (sub1 x) (sub1 y)))))

(define (o* x y)
  (cond ((= 0 y) 0)
        ((= 1 y) x)
        (else (o+ x (o* x (sub1 y))))))

(define (o/ x y)
  (cond ((= 0 y) null)
        ((= 1 y) x)
        ((< x y) 0)
        (else (add1 (o/ (o- x y) y)))))