Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/xmltest/valid/sa/045.xml
<!DOCTYPE doc [ <!ELEMENT doc (#PCDATA)> <!ATTLIST doc a1 CDATA "v1"> <!ATTLIST doc a1 CDATA "z1"> ]> <doc></doc>