Ensures :attr:`subject` is an iterable containing only objects of class *prototype*.

def is_an_iterable_of(self, prototype):
        """
        Ensures :attr:`subject` is an iterable containing only objects of class *prototype*.
        """
        for element in self._subject:
            self._run(unittest_case.assertIsInstance, (element, prototype))
        if isinstance(self._subject, Mapping):
            return MappingInspector(self._subject)