WS-Policy represents a set of specifications that describe the capabilities and constraints of the security (and other business) policies on intermediaries and end points (for example, required security tokens, supported encryption algorithms, and privacy rules) and how to associate policies with services and end points.
Two "operators" (XML tags) are used to make statements about policy combinations: Logically, an empty wsp:All tag makes no assertions.
However, synonymous assertions are considered incompatible by a policy intersection.
This can easily be explained by the fact that policy intersection is a syntactic approach, which does not incorporate the semantics of the assertions.
Opposed to what the name might suggest, a policy intersection is (although quite similar) not a set-intersection.