Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/oasis/p49pass1.xml
<!DOCTYPE doc [ <!ELEMENT doc ANY> <!--NOTE: XML doesn't specify whether this is a choice or a seq--> <!ELEMENT a (doc?)> <!ELEMENT b (doc|a)> <!ELEMENT c ( doc | a | c? )> ]> <doc/>