I have noticed that cxml is looping for a certain combination of xml/dtd. It loops in the recurse function inside compile-content-model in xml-parse.lisp. I cannot fix it myself but I am hoping that somebody else can find a solution using my minimal reproducible case, which can be found at https://doremir.com/files/cxml/xml-test.zip <https://doremir.com/files/cxml/xml-test.zip>. The xml file from the case looks like this: <?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE a PUBLIC "" ""> <a> <c/> </a> And the dtd file looks like this: <!ELEMENT b EMPTY> <!ELEMENT c EMPTY> <!ELEMENT a ((b?)*, c?)> The real world case is from MusicXML parsing and the problematic xml excerpt looks like this: <sound tempo="90"> <offset sound="yes">8</offset> </sound> And the corresponding dtd excerpt looks like this: <!ELEMENT sound ((midi-device?, midi-instrument?, play?)*, offset?)> I have verified this using version "cxml-2008-11-30" from the Closure XML Parser home page at https://common-lisp.net/project/cxml/index.html and also with the latest version from Quicklisp "cxml-20181018". By the way, why isn't the latest version downloadable from the Closure XML Parser home page? Thanks! Sven