Here is the DTD specified by the USPTO, which specifies optional fields and :
<?xml version="1.0" encoding="utf-8"?> <br> <!DOCTYPE us-patent-assignments [<!ELEMENT us-patent-assignments (action-key-code, transaction-date, patent-assignments)> <br> <!ATTLIST us-patent-assignments dtd-version CDATA #IMPLIED <br> date-produced CDATA #IMPLIED> <br>
<!ELEMENT action-key-code (#PCDATA)> <br>
<!ELEMENT transaction-date (date)><br>