Concrete Proposal of ConditionReference (#7)

This is a concrete proposal on new work item #7: Condition Reference.
The proposal is based on the generic variable reference approach by Polar.
Any comments are welcome.

- The following example describes an outline of the new policy statement
using <VariableDef> and <VariableRef> elements.

<Policy>
  <VariableDef VariableId="var1">
    ... a variable definition ...
  </VariableDef>
  <VariableDef VariableId="var2">
    ... a variable definition ...
  </VariableDef>
  <Rule Effect = "Permit">
    <Target> ... target 1
    <VariableRef VariableId="var1"/>
  </Rule>
  <Rule Effect = "Permit">
    <Target>... target 2
    <Condition FunctionId= "string-equal">
      <VariableRef VariableId="var1"/>
      <VariableRef VariableId="var2"/>
    </Condition>
  </Rule>
</Policy>

- I made several modifications on the current XACML policy schema.

Modifications are:
  + Create VariableDef element definition
  + Create VariableDef reference below PolicyType
  + Create VariableRef definition
  + Create VariableRef reference in ApplyCoreType
  + Modify Condition and ApplyType definition

- Sample XACML policy and the modified policy schema definition.
(the policy is valid against the new schema)
(See attached file: draft-xacml-schema-policy-v2-1.xsd)(See attached file:
XACMLPolicyRule1.xml)

Best,
Michiharu