[xml] RelaxNG schema: possible to put reference into XML file?

[David Hagood <wowbagger sktc net>]

It it true that there is no way to specify within an XML file that the
file has a specific RelaxNG schema associated with it? That is, that
unlike a DTD, it is NOT possible to have an XML file provide a reference
to a RelaxNG schema such that validating tools (e.g. libXML2) can
automatically find that schema and use it to validate the document.

I just want to verify my understanding of this.