<https://maspypy.com/projecteuler%e6%84%9f%e6%83%b3-%ef%bd%9e2019-08-03#article>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://maspypy.com/projecteuler%E6%84%9F%E6%83%B3-%EF%BD%9E2019-08-03>