Query on the XML graph

I am now doing the research on the XML graph. What I am wondering about is
that how the nowaday query engines cope with the XML files that have idref
attributes, or XML graph? Since XPath have no notation for the idref/id
edge. Or I miss some details?  So can I always treat the XML document as
a tree with the special node, i.e. idref node?

Thank you in advance for your suggestion.

Regards,
Fianny