See 

https://github.com/OpenGridForum/DFDL/pull/55/files#diff-c18a5ebbb168df129b8a9c8044835342d3deba6f6cc7029e08c45aea31a7c85e

Mike Beckerle 
Apache Daffodil PMC | daffodil.apache.org
OGF DFDL Workgroup Co-Chair | www.ogf.org/ogf/doku.php/standards/dfdl/dfdl
Owl Cyber Defense | www.owlcyberdefense.com