def find_null_net_stations(db, collection="arrival"):
    """
    Return a set container of sta fields for documents with a null net
    code (key=net).  Scans collection defined by collection argument.
    """
    dbcol = db[collection]
    net_not_defined = set()
    curs = dbcol.find()
    for doc in curs:
        if not 'net' in doc:
            sta = doc['sta']
            net_not_defined.add(sta)
    return net_not_defined