The identification information (typically the patent number, or number and name) for a cited patent
<!ELEMENT patent (#PCDATA %patent-elements;)* >
Any combination of:
<citation> Citation; <product> Product Information; <related-article> Related Article Information
<article>
<front>...
</front>
<body>...
</body>
<back>
<ref-list>
...
<ref>
<citation citation-type="patent">
<person-group person-group-type="inventor">
<name>
<surname>Blanco</surname>
<given-names>Ernesto E</given-names>
</name>
<name>
<surname>Meade</surname>
<given-names>John C</given-names>
</name>
<name>
<surname>Richards</surname>
<given-names>William D</given-names>
</name>
</person-group>
<collab collab-type="assignee">Ophthalmic Ventures</collab>
<article-title>Surgical Stapling system</article-title>
<source>US patent</source>
<patent>4,969,591</patent>
<year>1990</year>
<month>11</month>
<day>13</day>
</citation>
</ref>
</ref-list>
</back>
</article>
references.ent