code: public EvaluationResult getSubjectAttribute(URI type, URI id,
                                                  URI category) {
          return getSubjectAttribute(type, id, null, category);
      }
docs: Returns attribute value(s) from the subject section of the request
  that have no issuer.
  
  @param type the type of the attribute value(s) to find
  @param id the id of the attribute value(s) to find
  @param category the category the attribute value(s) must be in
  
  @return a result containing a bag either empty because no values were
  found or containing at least one value, or status associated with an
  Indeterminate result