<https://kei-hiramatsu.com/2010/10/16/%e6%98%8e%e6%97%a5%e3%81%8b%e3%82%89%e6%b2%96%e7%b8%84%e3%81%a7%e3%81%99%e3%80%82/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://kei-hiramatsu.com/2010/10/16/%E6%98%8E%E6%97%A5%E3%81%8B%E3%82%89%E6%B2%96%E7%B8%84%E3%81%A7%E3%81%99%E3%80%82/>