namespace obj_reader {
    struct V3
    {
        double x;
        double y;
        double z;
    };

    struct V4
    {
        double x;
        double y;
        double z;
        double w;
    };

    namespace parser
    {
        x3::rule<class obj_v3, V3>
            obj_v3 = "obj_v3";

        x3::rule<class obj_v4, V4>
            obj_v4 = "obj_v4";

        x3::rule<class obj_comment, string>
            obj_comment = "obj_comment";

        auto const obj_v3_def =
            'v' >> x3::double_ >> x3::double_ >> x3::double_;

        auto const obj_v4_def =
            'v' >> x3::double_ >> x3::double_ >> x3::double_ >> x3::double_;

        auto const obj_comment_def =
            '#' >> x3::lexeme[*x3::char_];

        BOOST_SPIRIT_DEFINE(obj_v3, obj_v4, obj_comment);
    }
}

BOOST_FUSION_ADAPT_STRUCT(
    obj_reader::V3,
    (double, x),
    (double, y),
    (double, z)
)

BOOST_FUSION_ADAPT_STRUCT(
    obj_reader::V4,
    (double, x),
    (double, y),
    (double, z),
    (double, w)
)