Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/9x9/xmlData21F.xml
<!DOCTYPE doc [ <!ELEMENT doc (#PCDATA)> <!ATTLIST doc a1 CDATA "v1"> <!ATTLIST doc a2 CDATA #IMPLIED> <!ATTLIST doc a2 CDATA "v2"> ]> <doc/>