def resource():
    """
        The resource which would appear at the root of the domain.  For
        example, if you were mocking an API typically present at
        ``https://api.foo.example.com/v2/``, this should return a resource
        whose ``.getChild("v2", ...)`` returns the root of the API.

        :return: The resource for the root of the API implemented by this
                 :obj:`IAPIDomainMock`.
        :rtype: :obj:`twisted.web.iweb.IResource`
        """
