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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_pretrained_file_names(cls, pretrained_file_name):
 """Checks if a pre-trained token embedding file name is valid. Parameters pretrained_file_name : str The pre-trained token embedding file. """


        embedding_name = cls.__name__.lower()
        if pretrained_file_name not in cls.pretrained_file_name_sha1:
            raise KeyError('Cannot find pretrained file %s for token embedding %s. Valid '
                           'pretrained files for embedding %s: %s' %
                           (pretrained_file_name, embedding_name, embedding_name,
                            ', '.join(cls.pretrained_file_name_sha1.keys())))