<https://crystal-verry.com/%e3%82%ab%e3%83%ab%e3%82%bb%e3%83%89%e3%83%8b%e3%83%bc%e7%99%bd%e3%81%84%e9%b3%a5%e3%80%80%e5%a4%a9%e7%84%b6%e7%9f%b3>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://crystal-verry.com/%E3%82%AB%E3%83%AB%E3%82%BB%E3%83%89%E3%83%8B%E3%83%BC%E7%99%BD%E3%81%84%E9%B3%A5%E3%80%80%E5%A4%A9%E7%84%B6%E7%9F%B3>