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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 check_extensions(extensions: Set[str], allow_multifile: bool = False):
 """ Utility method to check that all extensions in the provided set are valid :param extensions: :param allow_multifile: :return: """

    check_var(extensions, var_types=set, var_name='extensions')

    # -- check them one by one
    for ext in extensions:
        check_extension(ext, allow_multifile=allow_multifile)