constexpr void set_separator(basic_string_view<charT> sep) noexcept;
constexpr void set_brackets(basic_string_view<charT> opening,
                            basic_string_view<charT> closing) noexcept;
template<class ParseContext>
  constexpr typename ParseContext::iterator
    parse(ParseContext& ctx);
template<class FormatContext>
  typename FormatContext::iterator
    format(maybe-const-r& elems, FormatContext& ctx) const;