<https://larryullman.com/forums/index.php?/topic/2378-view_cartphp-chapter-19/#comment-16179>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://larryullman.com/forums/index.php?/topic/2378-view_cartphp-chapter-19/&tab=comments#comment-14846>