return fv(first_, last_, i) - x_;
root_type z = decimal_root (-0.99999 ,1000.0 ,bias_lower ,decimals_ ,*this ); if(root_not_bracketed == z.second) {  z.first = -1.0L; } return z.first;
    long double operator()(long double i)        {        return fv(first_, last_, i) - x_;        }