'Set the OpenID namespace URI used in this message.
@raises InvalidOpenIDNamespace: if the namespace is not in
L{Message.allowed_openid_namespaces}'

def setOpenIDNamespace(self, openid_ns_uri, implicit):

    if (openid_ns_uri not in self.allowed_openid_namespaces):
        raise InvalidOpenIDNamespace(openid_ns_uri)
    self.namespaces.addAlias(openid_ns_uri, NULL_NAMESPACE, implicit)
    self._openid_ns_uri = openid_ns_uri
