def connectionLost(reason):
    """
        Called when the connection is shut down.

        Clear any circular references here, and any external references
        to this Protocol.  The connection has been closed. The C{reason}
        Failure wraps a L{twisted.internet.error.ConnectionDone} or
        L{twisted.internet.error.ConnectionLost} instance (or a subclass
        of one of those).

        @type reason: L{twisted.python.failure.Failure}
        """
