proto::or_ (and I presume, but not verified, proto::and_) requires at least 2 arguments. It might be nice to allow 1 argument (and maybe 0 arguments as well). Zero and one can happen while playing around during development, but more importantly as the result of invoking other metafunctions computing grammars. Whether it is really an issue in practice, I cannot tell. For now I've hit this in the 'playing around' part, haven't needed to compute grammars yet. At least with GCC, the error you presently get when you try proto::or_ with one argument is that it requires 8 arguments, with no indication that anything between 2 and 8 would work. Arguably GCC error messages could be improved to take into consideration default aruments, but this is what 4.1.2 does. Best regards, Maurizio