@abstractmethod<EOL><INDENT>def login(self, auth_token):<DEDENT>
pass<EOL>
Performs a login attempt for this Subject/user.

If unsuccessful, a subclass of AuthenticationException is raised,
identifying why the attempt failed.

If successful, the Account data associated with the submitted
identifiers/credentials will be associated with this Subject and the
method will return quietly.

Upon returning quietly, this Subject instance can be considered
authenticated and its identifiers attribute will be non-None and
its authenticated property will be True.

:param authc_token: the token encapsulating the subject's identifiers
                    and credentials to be passed to the Authentication
                    subsystem for verification
:type authc_token:  authc_abcs.AuthenticationToken

:raises AuthenticationException: if the authentication attempt fails