boost::log::expressions::format_named_scope
// In header: <boost/log/expressions/formatters/named_scope.hpp> template<typename DescriptorT, template< typename > class ActorT, typename CharT> format_named_scope_actor< fallback_to_none, CharT, ActorT > format_named_scope(attribute_keyword< DescriptorT, ActorT > const & keyword, std::basic_string< CharT > const & element_format);