progenitor as hierarchical attribute

Consider defining an attribute like:

	urn:oasis:names:tc:xacml:3.0:resource:resource-progenitor

to refer to the hierarchical node at the far end of an ordered ancestor
list of non-XML resources.

This would be the non-XML analog to document-id for XML resources.

We have a use case in which a policy rule refers to the
resource-progenitor, without respect to any other part of the hierarchy.
For convenience and efficiency we would like to refer to a standard
attribute defined like this.

Thanks,
--Paul