<https://masato-k.com/2017/05/19/%e3%83%ad%e3%83%bc%e3%83%9e%e5%b8%82%e5%86%85%e4%ba%a4%e9%80%9a%e4%b8%80%e6%97%a5%e4%b8%ad/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://masato-k.com/2017/05/19/%E3%83%AD%E3%83%BC%E3%83%9E%E5%B8%82%E5%86%85%E4%BA%A4%E9%80%9A%E4%B8%80%E6%97%A5%E4%B8%AD/>