Ensures :attr:`subject` is an object of class *prototype*.

def is_a(self, prototype):
        """
        Ensures :attr:`subject` is an object of class *prototype*.
        """
        self._run(unittest_case.assertIsInstance, (self._subject, prototype))
        if hasattr(self._subject, '__iter__'):
            return IterableInspector(self._subject)
        else:
            return ReferenceInspector(self._subject)