'Add an acceptable authentication policy URI to this request
This method is intended to be used by the relying party to add
acceptable authentication types to the request.
@param policy_uri: The identifier for the preferred type of
authentication.
@see: http://openid.net/specs/openid-provider-authentication-policy-extension-1_0-01.html#auth_policies'

def addPolicyURI(self, policy_uri):

    if (policy_uri not in self.preferred_auth_policies):
        self.preferred_auth_policies.append(policy_uri)
