code: def check(self, value, namespace):
          
          if (not issubclass(type(value), self._cls) or
              len(value) != len(self._cls._fields)):
              return False
          for i, check in enumerate(self._checks):
              if not check(value[i], namespace):
                  return False
          return True
docs: Attribute _field_types is a dict from field name to type.