<https://nayukisnow.pixnet.net/blog/post/31801766-%e3%80%90%e9%81%8a%e3%80%91%e6%be%8e%e6%b9%96%e7%95%a2%e6%97%854%e6%97%a5%e9%81%8a%28day2-%e5%90%89%e8%b2%9d%e6%b0%b4%e4%b8%8a%e6%b4%bb%e5%8b%95%29>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://nayukisnow.pixnet.net/blog/post/31801766>