double &LabeledSales::operator [](int i) { if (i < 0 || i >= MONTHS) throw nbad_index(Label(), i); return Sales::operator [](i); }
double LabeledSales::operator[](int i) const throw(std::logic_error) { if(i < 0 || i >= MONTHS) throw nbad_index(Label(), i); return Sales::operator[](i); }