support new style divergence tester whose explain() returns a single value.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31070 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/Rule.py

@@ -216,7 +216,10 @@ class Rule(Predicate, XMLObject):
     for divergence_tester in self.contentValues(
                portal_type=self.getPortalDivergenceTesterTypeList()):
       result = divergence_tester.explain(sim_mvt)
-      result_list.extend(result)
+      if isinstance(result, (list, tuple)): # for compatibility
+        result_list.extend(result)
+      elif result is not None:
+        result_list.append(result)
     return result_list
 
   # XXX getSolverList is not part of the API and should be removed.