boost::proto::convertible_to — For matching terminals that are convertible to a type.
template<typename T> struct convertible_to { };
Use proto::convertible_to<> to match a terminal that is convertible to some type. For example, the grammar proto::terminal<proto::convertible_to<int> > will match any terminal whose argument is convertible to an integer.
proto::convertible_to<>
proto::terminal<proto::convertible_to<int> >