X3: How should a custom parser be implemented?

What interface should a custom parser expose? As of now, I implement my parsers like this:

struct custom_parser_type : x3::parser<custom_parser_type>
{
    static bool const handles_container = false;
    static bool const is_pass_through_unary = false;
    static bool const has_action = false;

    template<typename It, typename Ctx, typename RCtx, typename Attribute>
    bool parse(It& f, It l, Ctx&, RCtx&, Attribute& attr) const
    {
        // custom parsing here
        If (/*succeeded*/)
        {
            if constexpr (!std::is_same_v<Attribute, x3::unused_type>)
            {
                // set attribute value
                attr = {};
            }
            return true;
        }
        return false;
    }
};
inline const custom_parser_type custom_parser;

Is my implementation correct? Is it the full interface X3 parser has to provide? What's the purpose of three static bools? A sequence of custom parsers doesn't compile without has_action. Should I derive from x3::parser_base or x3::parser<Derived>?