CD-1 issue #3: dynamic RequestContextPath

The issue number refers to the XLS-sheet found in this email:
http://lists.oasis-open.org/archives/xacml/200909/msg00013.html

The commenter proposes that the RequestContextPath of an attribute 
selector made a child element of the attribute selector, so that the 
xpath can be dynamically constructed using XACML expressions.

Note that making this change is non-trivial because XPath expressions 
are not simple strings. An xpath also relies on a context for namespace 
prefix resolution, so simple string functions do not work well for 
constructing xpath expressions. Adopting this proposal would also mean 
that we have to design a library of xpath manipulation functions.

I also have concerns about how this additional power to XACML would 
affect how XACML can be analyzed and audited, so I would not like to 
make this change without thinking it through carefully.

I propose that we reject this proposal for XACML 3.0 since I think we 
should wrap up 3.0 now. We can consider this for the future.

Best regards,
Erik