8. Unary member operator syntax

• 
      // rational.h
      class Rational {
          //...
      public:
          // Unary negation, as in: x = -y
          Rational operator-() const;
      };
  
      // rational.cpp
      Rational Rational::operator-() const
      {
          // uses 2-argument constructor
          return Rational( -m_n, m_d );
      }