The PolicyEngine can have a default PolicyRegistry that the Policy objects
that it creates are setup to use when normalize is called without the
PolicyRegistry.
Returns a normalized version of self.If deep is set
false then the assertions are not normalized and it returns a
partially normalized version of self.
Throws an UnsupportedOperationException since PolicyReference.normalize()
can't resolve the Policy that it refers to unless a PolicyRegistry is
provided.
Implementation of an assertion that required exactly one (possibly empty) child element
of type Policy (as does for examples the wsam:Addressing assertion).