(define (estValide? c a b) (not (or
                                 (and (eq?  (eval c) (eval /)) (= b 0))
                                 (< ((eval c) a b) 0)
                                 (not (integer? ((eval c) a b)))
                                )
                           )
)

(define (opere op x y) (cond
                         ((null? op) op)
                         ((estValide? (eval (car op)) x y) (cons ((eval (car op)) x y) (opere (cdr op) x y)))
                         ((estValide? (eval (car op)) y x) (cons ((eval (car op)) y x) (opere (cdr op) y x)))
                         (else (opere (cdr op) y x))
                       )
)