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<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://hitomi-ito.com/blogs/%E3%83%8B%E3%83%A5%E3%83%BC%E3%82%B9/%E6%96%B0%E8%81%9E%E6%8E%B2%E8%BC%89-%E6%9C%9D%E6%97%A5%E6%96%B0%E8%81%9E%E4%BA%AC%E9%83%BD%E7%89%88>