List Iterator Interface

public:
  // terminology support
  typedef T               ValueType;
  typedef ListIterator<T> Iterator;
  // operators
  bool      operator == (const Iterator& i2) const;
  bool      operator != (const Iterator& i2) const;
  Iterator& operator =  (const Iterator& i);
  T&        operator *  ();         // Return reference to current Tval
  const T&  operator *  () const;   // const version
  Iterator& operator ++ ();         // prefix
  Iterator  operator ++ (int);      // postfix
  Iterator& operator -- ();         // prefix
  Iterator  operator -- (int);      // postfix
  bool      Valid       () const;   // Iterator is (pointing to) a valid element