def AR_2(dul):
    """Association release AR-2.

    On receiving an association release request, send release indication

    State-event triggers: Sta6 + Evt12

    Parameters
    ----------
    dul : dul.DULServiceProvider
        The DICOM Upper Layer Service instance for the local AE

    Returns
    -------
    str
        ``'Sta8'``, the next state of the state machine
    """
    # Send A-RELEASE indication primitive
    dul.to_user_queue.put(dul.primitive)

    return 'Sta8'