code: def _make_retval_checker(self):
          
          rvchk = self.retval.checker
          if rvchk:
              def _checker(value):
                  if not rvchk.check(value):
                      raise self._type_error(
                          "Incorrect return type in %s: expected %s got %s" %
                          (self.name_bt, rvchk.name(),
                           checker_for_type(type(value)).name())
                      )
          else:
              def _checker(value):
                  pass
          return _checker
docs: Create a function that checks the return value of the function.