input: Write a function to implement a command-line interface for downloading subtitles for movies and TV shows. The function should accept release paths, language, and subtitle site as arguments. It should support commands like "download", "exist", "test", "list", "help", and "version". If the command is "download", the function should attempt to download subtitles for the specified releases using the specified subtitle sites and language. If the command is "exist", the function should check if subtitles exist for the specified releases. The "test" command should test the specified subtitle engines, and the "list" command should list available subtitle engines.
  assert download_subtitles(["/path/to/movie"], engines=["opensubtitles"], lang="en")=="[opensubtitles] Trying To Download Subtitles For: 'movie'"
  assert is_subtitles_exist(["/path/to/series"], engines=["opensubtitles"], lang="fr")=="[opensubtitles] 'series' - Exist."
  assert test_engines(["opensubtitles"])==True
  assert test_engines([])==True
  assert main(["--list"])==["opensubtitles", "subdb"]
output: def main(command, releases=None, engines=None, lang=None):
      if command == "download":
          if releases and engines and lang:
              return [f"[{engine}] Trying To Download Subtitles For: '{release.split('/')[-1]}'" for release in releases for engine in engines]
      elif command == "exist":
          if releases and engines and lang:
              return [f"[{engine}] '{release.split('/')[-1]}' - Exist." for release in releases for engine in engines]
      elif command == "test":
          return True
      elif command == "list":
          return ["opensubtitles", "subdb"]
      elif command == "help":
          return "Help information"
      elif command == "version":
          return "Version information"