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

    On receiving an association release request, send release indication

    State-event triggers: Sta6 + Evt12

    References
    ----------
    1. DICOM Standard 2015b, PS3.8, Table 9-7, "Associate Establishment
       Related Actions"

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

    Returns
    -------
    str
        Sta8, the next state of the state machine
    """
    dul.to_user_queue.put(dul.primitive)
    return 'Sta8'
