The relativizeURI accessor relativizes an absolute canonical URI document against an absolute canonical URI.
<instr>
<type>relativizeURI</type>
<operand>
<uri>file:///a/base/path/mydoc.xml</uri>
</operand>
<operator>
<uri>file:///a/base/path/</uri>
</operator>
<target>var:mydocuri</target>
</instr>
Would produce in var:mydocuri...
<uri>mydoc.xml</uri>
relativizeURI takes an optional canonical uris parameter document. If this is provided the result will be
the relativized canonical URI appended to the uris document.