<https://jref.com/threads/%E3%81%A6%E3%82%82%E3%82%89%E3%81%86-and-extra.51833/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://jref.com/threads/%E3%81%A6%E3%82%82%E3%82%89%E3%81%86-and-extra.51833/#post-721984>