Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/oasis/p58fail5.xml
<!DOCTYPE doc [ <!ELEMENT doc EMPTY> <!NOTATION a SYSTEM "a"> <!NOTATION b SYSTEM "b"> <!ATTLIST doc att NOTATION(a|b) #IMPLIED> ]> <doc/>