Title: Contradictory recommendation for nesting and intersection 

Description: The specification provides some guidance about when to include an empty policy element. Section 4.3.2, Assertion/Policy element states:

{Note: if the schema outline for an assertion type requires a nested policy expression but the assertion does not further qualify one or more aspects of the behavior indicated by the assertion type (i.e., no assertions are needed in the nested policy expression), the assertion MUST include an empty <wsp:Policy/> Element Information Item in its [children] property; as explained in Section 4.3.3 Policy Operators, this is equivalent to a nested policy expression with a single alternative that has zero assertions. The reason for requiring at least an empty <wsp:Policy/> Element above is to ensure that two assertions of the same type will always be compatible and an intersection would not fail (see Section 4.5 Policy Intersection). 
}
The paragraph stated imply two different and somewhat contradictory advice. 
(a) A nested assertion by definition (via schema) should always expressed using nesting, even if the nesting yields zero assertions in a single alternative. (This is per the first statement) 
(b) Two assertions that have the same nesting will always be compatible REGARDLESS of whether they have different nested elements inside. (This is inferred from the last statement). 
While the first statement is intuitive, the second recommendation (b) is counter indicative per the intersection algorithm and thus requires either changing or clarification. This is due to the unclarity of the intersection algorithm. Consider the following two policy expressions:
(1) 
<wsp:Policy>
      <wsp:ExactlyOne>
             <wsp:All>
                      <ex:NestedAssertion>
                                 <wsp:Policy>
                                          <ex:Foo/>
                                 </wsp:Policy>
                     </ex:NestedAssertion>
            </wsp:All>
      </wsp:ExactlyOne>
</wsp:Policy>
(2) 
<wsp:Policy>
      <wsp:ExactlyOne>
             <wsp:All>
                      <ex:NestedAssertion>
                                 <wsp:Policy/>
                     </ex:NestedAssertion>
            </wsp:All>
      </wsp:ExactlyOne>
</wsp:Policy>
According to the statement above, these assertions are expected to be "compatible" but the intersection algorithm in Section 4.5 does not confirm this expectation: 
{If either assertion contains a nested policy expression, the two assertions are compatible if they both have a nested policy expression and the alternative in the nested policy expression of one is compatible with the alternative in the nested policy expression of the other. }
According to the previous statement, the nested expressions in (1) and (2) are NOT compatible assertions, either in strict or lax mode because of the definition of the compatibity of the alternatives are governed by the following. (Lets consider strict mode for simplicity) 
{If the mode is strict, two policy alternatives A and B are compatible:
if each assertion in A is compatible with an assertion in B, and
if each assertion in B is compatible with an assertion in A.}
The alternative in (1) is <wsp:Policy> <ex:Foo/></wsp:Policy>. The alternative in (2) is <wsp:Policy/>. According to the compatibility definition, these two alternatives are not compatible as there is no nested ex:Foo element within the second alternative for (2). 
Therefore, including a nested policy expression WILL STILL FAIL the intersection algorithm in contradiction to the statement: 
{The reason for requiring at least an empty <wsp:Policy/> Element above is to ensure that two assertions of the same type will always be compatible and an intersection would not fail (see Section 4.5 Policy Intersection). }
Thus the specification is in conflict with itself and this should be resolved. See proposal section for two alternative ways of fixing this. 

Target: Framework, Primer


Justification: The specification is contradictory with itself. It does not explain the utility of nesting and empty policy expression well. The clarification should be included in the framework as well as the primer since it was deemed necessary for an explanation in the framework document itself for further clarification in the first place. Readers who are not familiar with the nesting will definitely get this wrong, especially there is contradictory statements in the specification. 

Proposal: 
There are two ways to interpret this conflict as there are two possible ways forward depending on the intent of the specification: 
(a) The statement in 4.3.2 quoted is in error. Including a nested empty policy expression allows the compatibility testing to occur, but does NOT guarantee the same types to be compatible for intersection (which is implied by the intersection algorithm). Using this logic, expressions 1 and 2 are not compatible as the intersection algorithm suggests. 
This requires fixing the last sentence in the quoted paragraph in Section 4.3.2. 
(b) The intersection algorithm makes a special provision for an empty policy assertion to allow compatibity with nesting. This means expressions 1 and 2 are always compatible with each other. This means when we have nested empty policies, it is a cop-out for cheating the intersection algorithm and thus requires the intersection algorithm to account for this specifically. 
The resolution requires including an example, preferably to the framework, alternatively to the primer to illustrate the result of intersection with the examples provided in this report. If (b) is chosen, guidance to Guidelines document will be appropriate in addition to the framework fix. 


[1] http://www.w3.org/TR/ws-policy/