def AA_6(dul):
    """Association abort AA-6.

    If receive a PDU while waiting for connection to close, ignore it

    State-event triggers: Sta13 + Evt3/Evt4/Evt10/Evt12/Evt13

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

    Returns
    -------
    str
        ``'Sta13'``, the next state of the state machine
    """
    # Ignore PDU
    dul.primitive = None

    return 'Sta13'