Dispatch handling of this event to a receiver.

        This method will invoke ``receiver._session_callback_removed`` if
        it exists.

def dispatch(self, receiver):
        ''' Dispatch handling of this event to a receiver.

        This method will invoke ``receiver._session_callback_removed`` if
        it exists.

        '''
        super(SessionCallbackRemoved, self).dispatch(receiver)
        if hasattr(receiver, '_session_callback_removed'):
            receiver._session_callback_removed(self)