16. Increment ++ and decrement -- syntax

• 
  // rational.h
  class Rational {
      //...
  public:
      Rational& operator--(); // prefix (result is an lvalue!)
      const Rational operator--(int); // postfix (not an lvalue)
  };
  
  // rational.cpp
  Rational& Rational::operator--()
  {
      m_n -= m_d;
      return *this;
  }
  
  const Rational Rational::operator--(int)
  {
      Rational temp = *this;
      m_n -= m_d;
      return temp;
  }