///\brief Definition of INVALID.

 ///\brief Definition of INVALID.

 namespace lemon {

 namespace lemon {

   /// \brief Dummy type to make it easier to make invalid iterators.

   /// \brief Dummy type to make it easier to make invalid iterators.

   /// See \ref INVALID for the usage.

   /// See \ref INVALID for the usage.

   struct Invalid {

   struct Invalid {

   public:

   public:

     bool operator==(Invalid) { return true;  }

     bool operator==(Invalid) { return true;  }

     bool operator!=(Invalid) { return false; }

     bool operator!=(Invalid) { return false; }