Load the model:
<xsl:variable name="model" select="rt:load('diagrams/bgraph.rdf')"/>
Grab a node:
<xsl:variable name="A" select="rt:resource('http://uri/for/A')"/>
Turn the results into a tree:
<xsl:variable name="tree" select="rt:twig($A)"/>
At this point, $tree contains an XML document that can be queried and transformed with XSLT like any other input document.