//Demonstrate how 3-ary tag::plus expression can be created.

#include <boost/proto/proto.hpp>
#include <boost/mpl/assert.hpp>

using namespace boost::proto;

typedef terminal<int>::type expr_int_type;

    typedef 
  nary_expr
  < tag::plus
  , _
  , _
  , _
  >
gram_plus3_type
;
    typedef 
  nary_expr
  < tag::plus
  , expr_int_type
  , expr_int_type
  , expr_int_type
  >
expr_plus3_type
;
BOOST_MPL_ASSERT((matches<expr_plus3_type,gram_plus3_type>));
    typedef 
  nary_expr
  < tag::plus
  , expr_int_type
  , expr_int_type
  >
expr_plus2_type
;
BOOST_MPL_ASSERT((matches<expr_plus2_type,gram_plus3_type>));//fails