template <int n>
   struct FACTORIAL_of_NEGATIVE_NUMBER
     : mpl::greater_equal<mpl::int_<n>, mpl::int_<0> >
   {};
   template <class N>
   struct factorial
     : mpl::eval_if<
           mpl::equal_to<N,mpl::int_<0> >
         , mpl::int_<1>
         , mpl::multiplies<
               N
             , factorial<typename mpl::prior<N>::type>
           >
       >
   {
       BOOST_MPL_ASSERT((FACTORIAL_of_NEGATIVE_NUMBER<N::value>));
   };
static const int fact_7 = factorial<7>::value;