code: def update_extra_fields(subtype, chan, num, aux_note, update):
      
  
      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
docs: 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.