Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/oasis/p47pass1.xml
<!DOCTYPE doc [ <!ELEMENT doc ANY> <!ELEMENT a (doc,a?)> <!ELEMENT b (doc|a)> <!ELEMENT c (a,b)?> <!ELEMENT d (a|b)? > <!ELEMENT e (a,b)* > <!ELEMENT f (a,b)+ > ]> <doc/>