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See XPath.html and the discussion on QNames in the class overview. Also see NamespaceContext.html for details on how to bind the default namespace.
Daffodil uses Saxon-B and constructs a namespace context object to provide resolution of prefixes for XPath.
The way this works, is that whenever we have a DFDL expression, we also have the encapsulating XML object that contained it. The namespace scope of that XML object defines what the prefixes in the expression mean. So we grab the namespace scope from the XML object, and massage it slightly to what Saxon-B Daffodil wants to provide those definitions.
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