[xacml] The use of Xpath from Concall

[Polar Humenn <polar@syr.edu>]

Polar made the point that we we probably want to say things
such as the principal that is the Doctor cannot be the same principal that
is the Pharmacist. So, we have to use "../" on the XPATH selection of the
"role" Attribute, but compare the names that are superior on the selection
path. (Please exuse my lack of Xpath specifics, but this is the rough
idea).

<notEquals>

<Principals/SimplePrincipal/Attribute[@AttributeName="Role" and
@AttributeValue="Doctor"]/../PrincipalName>

<Principals/SimplePrinipal/Attribute[@AttributeName="Role" and
@AttributeValue="Parmacist"]/../PrincipalName>

</notEquals>