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::address_of<proto::_> > {}; // Forward-declare an expression wrapper template<typename Expr> struct make_state; struct sm_domain : proto::domain< proto::generator<make_state>, state_grammar > {}; template<typename Expr> struct make_state : proto::extends<Expr, make_state<Expr>, sm_domain> { typedef proto::extends<Expr, make_state<Expr>, sm_domain> 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<proto::terminal<state_tag>::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&...