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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update_extra_fields(subtype, chan, num, aux_note, update):
 """ Update the field if the current annotation did not provide a value. - aux_note and sub are set to default values if missing. - chan and num copy over previous value if missing. """


    if update['subtype']:
        subtype.append(0)

    if update['chan']:
        if chan == []:
            chan.append(0)
        else:
            chan.append(chan[-1])
    if update['num']:
        if num == []:
            num.append(0)
        else:
            num.append(num[-1])

    if update['aux_note']:
        aux_note.append('')

    return subtype, chan, num, aux_note