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boost::proto::or_ — For matching one of a set of alternate grammars. Alternates are tried in order to avoid ambiguity.
          When used as a transform, proto::or_<> applies the transform
          associated with the first grammar that matches the expression.
// In header: <boost/proto/matches.hpp>
template<typename... G> 
struct or_ :  proto::transform<or_<G...> > {
  // types
  typedef or_ proto_grammar;
  // member classes/structs/unions
  template<typename Expr, typename State, typename Data> 
  struct impl :  proto::transform_impl< Expr, State, Data > {
    // types
    typedef unspecified result_type;
    // public member functions
    result_type operator()(typename impl::expr_param, 
                           typename impl::state_param, 
                           typename impl::data_param) const;
  };
};
            An expression type E matches
            proto::or_<G0,G1,...Gn>
            if E matches any Gx for
            x in [0,n].
          
            When applying
            proto::or_<G0,G1,...Gn>
            as a transform with an expression e of type E,
            state s and data d, it is equivalent to
            Gx()(e, s, d), where
            x is the lowest number such that
            proto::matches<E, Gx>::value
            is true.
          
            The maximun number of template arguments proto::or_<> accepts
            is controlled by the BOOST_PROTO_MAX_LOGICAL_ARITY
            macro.