Hi Eric, I need a context with something like the following methods: template< typename Tag, typename Left, typename Right > result_type operator()(expr<Tag, Left, Right> const &expr) const { std::cout<<std::setw(indent) <<""<<"binary_expr:"<<expr.get_instance()<<"\n"; indent+=2; proto::eval(left, *this); proto::eval(right, *this); indent-=2; } where the expression tree node argument remains "packaged": (expr<Tag, Left, Right> const &expr) in contrast to the unpacked version: (Tag, Left const &left, Right const &right) shown in, for example: http://boost-sandbox.sourceforge.net/libs/proto/doc/html/boost_proto/user_s_... IOW, the calculator.html page suggests an interface like: template< typename Tag, typename Left, typename Right > result_type operator()(Tag, Left const &left, Right const &right) const { std::cout<<std::setw(indent)<<""<<"binary_expr\n"; indent+=2; proto::eval(left, *this); proto::eval(right, *this); indent-=2; } where there's no access to the "packaged" extended node, expr<Tag,Left,Right>, where the extension is, for example: struct xmpl_domain; struct xmpl_delta { unsigned const my_instance; static unsigned our_instances; xmpl_delta(void) : my_instance(++our_instances) {} xmpl_delta(xmpl_delta const&) : my_instance(++our_instances) {} void operator=(xmpl_delta const&) {} unsigned get_instance(void)const { return my_instance; } }; unsigned xmpl_delta::our_instances=0; template<typename Expr> struct xmpl_expression : proto::extends<Expr, xmpl_expression<Expr>, xmpl_domain> , xmpl_delta { ... }; I've looked at the formated output of the preprocessed context.hpp; however, I'm still having difficulty figuring out if this "packaged" method interfaces is possible. Any help is appreciated. TIA. -regards, Larry