Hi, I have a grammar, domain and an extended expression i.e. struct ga_domain : proto::domain, ga_grammar> {}; template<typename Expr> struct ga_expr : boost::proto::extends { // .... }; When the expression is transformed some instances of the ga_expr<> wrapper get removed. For example // some function that expects a ga_expr<Expr> as 2nd arg void evaluate_expr(Res& res, const ga_expr<Expr>& expr) { // .... } // some function which transforms the expression and calls evaluate_expr() template void evaluate(Res& res, const ga_expr<Expr>& expr) { BOOST_MPL_ASSERT((proto::matches< ga_expr<Expr>, ga_grammar>)); evaluate_expr(res, ET::transform_expression()(expr) ); } With the (do nothing) transform struct transform_expression : proto::nary_expr > {}; I get the following messages (using gcc 4.4.1, boost 1.42) In function ‘void clifford::evaluate(Res&, const clifford::ga_expr<Expr>&) [with Res = clifford::element >, clifford::cga3::cga3, clifford::default_tags>, Expr = boost::proto::exprns_::expr, mpl_::integral_c >, clifford::cga3::cga3, clifford::default_tags>&>, 0l> > >, 1l>]’: error: no matching function for call to ‘evaluate_expr(clifford::element >, clifford::cga3::cga3, clifford::default_tags>&, boost::proto::exprns_::expr, mpl_::integral_c >, clifford::cga3::cga3, clifford::default_tags>&>, 0l> >&>, 1l>&)’ So the clifford::ga_expr<> wrapper got removed from the unary reverse operator and i'm not sure what i'm supposed to add to make it stay. thanks