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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 vtt_talk_sources(ttFont) -> List[str]: """Return the tags of VTT source tables found in a font."""

  VTT_SOURCE_TABLES = {'TSI0', 'TSI1', 'TSI2', 'TSI3', 'TSI5'}
  tables_found = [tag for tag in ttFont.keys() if tag in VTT_SOURCE_TABLES]
  return tables_found