Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/oasis/p51pass1.xml
<!DOCTYPE doc [ <!ELEMENT doc (#PCDATA)> <!ELEMENT a (#PCDATA|doc)*> <!ELEMENT b ( #PCDATA | doc | a | b )*> <!ELEMENT c (#PCDATA)*> ]> <doc/>