(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))
                       )
)