//The following modifed from:
/*
From: "Robert Ramey" <ramey@rrsd.com>
Newsgroups: gmane.comp.lib.boost.user
Subject: challange to MPL gurus
Date: Mon, 12 Dec 2011 11:41:09 -0800
 */
#include <boost/mpl/list.hpp>
#include <boost/mpl/deref.hpp>
#include <boost/mpl/next.hpp>
#include <boost/mpl/begin_end.hpp>
#include <boost/mpl/if_recur.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/arg.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/type_traits/is_convertible.hpp>
#include <boost/concept/assert.hpp>

namespace boost{namespace mpl{

template<typename S>
struct ForwardSequence 
{
        typedef 
      typename end<S>::type 
    iter_end;
    //#define EMPTY_SEQ
        typedef
    #ifndef EMPTY_SEQ
      typename begin<S>::type 
    #else
      iter_end
    #endif
    iter_beg;
    BOOST_MPL_ASSERT
    (( is_convertible
       < typename iter_beg::category
       , forward_iterator_tag
       >
    ));
      template
      < typename StateIter
      >
    struct next_deref
    : next<StateIter>
    {
            typedef typename 
          deref<StateIter>::type 
        is_deref_able
          //check that StateIter is deref-able.
          ;
      //#define FAIL_ITER_CHECK
      #ifdef FAIL_ITER_CHECK
     private:
        struct my_private_t{};
        BOOST_MPL_ASSERT
        (( is_same
           < is_deref_able
           , my_private_t
           >
        ))
          //Just to provide example test on 
          //is_deref_able to test that
          //this template is instantiated.
          //This should always fail.
          ;
      #endif
    };
        typedef typename
      if_recur
      < identity<iter_beg>
      , not_
        < boost::is_same
          < arg<1>
          , iter_end
          >
        >
      , next_deref<arg<1> >
      >::type
    type;
};

void test_list_sequence(){
    BOOST_CONCEPT_ASSERT(( ForwardSequence<
            list< int, char>
        >
    ));
} 

}}

int main()
{
    boost::mpl::test_list_sequence();
    return 0;
}