code: def unmarshal_props(self, fp, cls=None, id=None):
          
          if isinstance(fp, str) or isinstance(fp, unicode):
              doc = parseString(fp)
          else:
              doc = parse(fp)
          return self.get_props_from_doc(cls, id, doc)
docs: Same as unmarshalling an object, except it returns
          from "get_props_from_doc"