code: def define_help_flags():
    
    # Use a global to ensure idempotence.
    global _define_help_flags_called
  
    if not _define_help_flags_called:
      flags.DEFINE_flag(HelpFlag())
      flags.DEFINE_flag(HelpshortFlag())  # alias for --help
      flags.DEFINE_flag(HelpfullFlag())
      flags.DEFINE_flag(HelpXMLFlag())
      _define_help_flags_called = True
docs: Registers help flags. Idempotent.