An XmlResolver, which will be used to resolve URIs of documents being loaded and of references to external entities within those documents.
By default an XmlUrlResolver is used. This means that the responsibility
for resolving and dereferencing URIs rests with the .NET platform (and not with the
GNU Classpath).
When Saxon invokes a user-written XmlResolver, the GetEntity method
may return any of: a System.IO.Stream; a System.IO.TextReader; or a
java.xml.transform.Source.