<https://news.gandi.net/ja/2021/06/why-mx-records-have-a-priority-and-a-and-ns-records-dont>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://news.gandi.net/ja/2021/06/why-mx-records-have-a-priority-and-a-and-ns-records-dont/>