So.... I have a function that returns an expression template of proto::tag::function type, like so: template <class Exp>\ typename proto::result_of::make_expr<\ proto::tag::function\ , detail::BOOST_JOIN(func, _funct)<typename detail::backend_type<Exp>::type>\ , detail::big_number_exp<Exp>\

::type const \ func(const detail::big_number_exp<Exp>& arg)\ {\ return proto::make_expr<proto::tag::function>(\ detail::BOOST_JOIN(func, _funct)<typename detail::backend_type<Exp>::type>() \ , arg \ );\ }\

and it all works fine, until I try and use the result of such a function call in an expression: sqrt(arg) + arg yeilds: 1>m:\data\boost\sandbox\big_number\libs\math\ide\extended_real\scrap\scrap.cpp(25): error C2893: Failed to specialize function template 'const boost::proto::detail::enable_binary<boost::proto::domainns_::deduce_domain,boost::proto::detail::not_a_grammar,boost::mpl::or_<boost::proto::is_extension<T>,boost::proto::is_extension<Right>>,boost::proto::tag::plus,const Left,const Right>::type boost::proto::exprns_::operator +(const Left &,const Right &)' 1> With the following template arguments: 1> 'boost::math::detail::big_number_exp<Expr>' 1> with 1> [ 1> Expr=boost::proto::exprns_::basic_expr<boost::proto::tag::function,boost::proto::argsns_::list2<boost::math::detail::big_number_exp<boost::proto::exprns_::basic_expr<boost::proto::tag::terminal,boost::proto::argsns_::term<boost::math::detail::sqrt_funct<boost::math::mpfr_real_backend<50>>>,0>>,boost::math::detail::big_number_exp<boost::proto::exprns_::expr<boost::proto::tag::terminal,boost::proto::argsns_::term<boost::math::big_number<boost::math::mpfr_real_backend<50>> *>,0>>>,2> 1> ] 1> 'boost::math::mpfr_real_50' So I figured I needed to update my grammar, so I tried adding this case: template<> struct big_number_grammar_cases::case_<proto::tag::function> : proto::function< proto::_ > {}; But still no joy (same error message). Any ideas on what I'm doing wrong? BTW I couldn't find any examples of using proto::function in a grammar, it wasn't obvious to me what it's template arguments should be, or indeed whether this actually enabled the function call operator rather than my use case? Thanks, John.