Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/ibm/not-wf/P50/ibm50n03.xml
<?xml version="1.0"?> <!DOCTYPE root [ <!ELEMENT root (#PCDATA)> <!ELEMENT a ANY> <!ELEMENT b ANY> <!--* wrong separators in seq *--> <!ELEMENT aElement (a,a|b) > ]> <root>Any content</root>