I've defined some custom tag for user specific function like : struct conj_tag {}; I have made the following expression builder using make_expr : template<class X> typename proto::result_of::make_expr< conj_tag, X const &>::type conj( X const& x ) { return proto::make_expr<conj_tag>(boost::cref(x)); } And I match this function using the following grammar rule : struct my_grammar : or_< // some terminal , bp::function< bp::terminal<conj_tag>, my_grammar> > {}; Now, how should I overload operator() of a callable_context to match this special tag ? Thanks in advance -- ___________________________________________ Joel Falcou - Assistant Professor PARALL Team - LRI - Universite Paris Sud XI Tel : (+33)1 69 15 66 35