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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 set_global(cls, user_agent=None, user_agent_config_yaml=None, user_agent_lookup=None, **kwargs):
 # type: (Optional[str], Optional[str], Optional[str], Any) -> None """ Set global user agent string Args: user_agent (Optional[str]):
 User agent string. HDXPythonLibrary/X.X.X- is prefixed. user_agent_config_yaml (Optional[str]):
 Path to YAML user agent configuration. Ignored if user_agent supplied. Defaults to ~/.useragent.yml. user_agent_lookup (Optional[str]):
 Lookup key for YAML. Ignored if user_agent supplied. Returns: None """

        cls.user_agent = cls._create(user_agent, user_agent_config_yaml, user_agent_lookup, **kwargs)