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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 default(cls):
 """Returns the default set of jar rules. Can be set with `set_default` but otherwise defaults to `skip_signatures_and_duplicates_concat_well_known_metadata`. :API: public """

    if cls._DEFAULT is None:
      cls._DEFAULT = cls.skip_signatures_and_duplicates_concat_well_known_metadata()
    return cls._DEFAULT