Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/oasis/p09pass1.dtd
<!ELEMENT doc EMPTY> <!ENTITY % ent1 ""> <!ENTITY ent2 "text2"> <!ENTITY % ent3 "<!-- <!DOCTYPE <!ELEMENT <? '''"&ent2; %ent1;"> <!ENTITY % ent4 '""''"'>