Thanks to Eric and Joel, it now works. Just for info, the solution: // grammar forbidding address of for terminals struct state_grammar : proto::not_proto::_ > {}; // Forward-declare an expression wrapper template<typename Expr> struct make_state; struct sm_domain : proto::domain< proto::generator, state_grammar > {}; template<typename Expr> struct make_state : proto::extends { typedef proto::extends base_type; // Needs a constructor make_state(Expr const &e = Expr()) : base_type(e) {} // Unhide Proto's overloaded assignment operator using base_type::operator=; }; Then, to define a terminal: struct s1: make_state::type>{}; Thanks a lot! Regards, Christophe _________________________________________________________________ Invite your mail contacts to join your friends list with Windows Live Spaces. It's easy! http://spaces.live.com/spacesapi.aspx?wx_action=create&wx_url=/friends.aspx&mkt=en-us