def AR_10(dul):
    """Association release AR-10.

    On receiving A-RELEASE-RP, issue release confirmation

    State-event triggers: Sta10 + Evt13

    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
        Sta12, the next state of the state machine
    """
    dul.to_user_queue.put(dul.primitive)
    return 'Sta12'
