Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/bugs/erik.dtd
<!DOCTYPE erik [ <!ELEMENT kenmerk (#PCDATA | begin | eind)*> <!ATTLIST kenmerk kmc CDATA #REQUIRED gewicht CDATA "1" > <!ELEMENT begin (#PCDATA)*> <!ELEMENT eind (#PCDATA)> ]>