<https://bluetreebooks.com/product/%e5%91%aa%e8%a1%93%e5%bb%bb%e6%88%a6-7/#product>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://bluetreebooks.com/product/%E5%91%AA%E8%A1%93%E5%BB%BB%E6%88%A6-7/>