Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/ibm/valid/P37/ibm37v01.xml
<!DOCTYPE book [ <!ELEMENT book ANY> <!ATTLIST book xml:lang CDATA #REQUIRED> ]> <book xml:lang="x-uk-eng">It is written in English</book>