Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/xmltest/not-wf/sa/075.xml
<!DOCTYPE doc [ <!ENTITY e1 "&e2;"> <!ENTITY e2 "&e3;"> <!ENTITY e3 "&e1;"> ]> <doc a="&e1;"></doc>