(define-class (car mileage tank)
    (instance-vars (gas 0))
    (default-method '(Unknown car command)) ;;NEW

    (method (drive miles)
         (let ((gas-needed (/ miles mileage)))
             (if (< gas-needed gas)
                     (set! gas (- gas gas-needed))
                     '(Not enough gas))))
    (method (refill)
         (set! gas tank))
)


(define new-car (instantiate car 20 40))