Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/ibm/invalid/P50/ibm50i01.xml
<?xml version="1.0"?> <!DOCTYPE root SYSTEM "ibm50i01.dtd" [ <!ELEMENT root (a,b)> ]> <root><a/><b> <c></c > content of b element </b></root> <!--* a invalid test: tests VC:Proper Group/PE Nesting in P50 *-->