(arith-exp (op args) (calc-arith op (map
                                                  (lambda (arg) (value-of arg env)) args
                                                  )))
(define calc-arith
  (lambda (op-struct args)
    (let* ([op (extractOperator op-struct)]
           [resolved-args (deref-list args)])
      (cond ((has-undefined? args) 'undefined)
            ((has-nonint? args) 'undefined)
            (else
              (if (null? (cdr resolved-args))
                  (op (get-identity op) (car resolved-args))
                  (fold-left op (car resolved-args) (cdr resolved-args))))))))