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

    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
        Sta13, the next state of the state machine
    """
    dul.primitive = None
    return 'Sta13'
