(let (binop? (lambda (expr) (if (pair? expr) ;; z racji chwilowego braku 'and' / 'andmap' uzywam condow
                                             (cond
                                               [(eq? '+ (car expr)) true]
                                               [(eq? '- (car expr)) true]
                                               [(eq? '* (car expr)) true]
                                               [(eq? '/ (car expr)) true]
                                               [true false])
                                             false)))