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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_source(cls, source_file_hash, source):
 """Checks if a pre-trained token embedding source name is valid. Parameters source : str The pre-trained token embedding source. """

        embedding_name = cls.__name__.lower()
        if source not in source_file_hash:
            raise KeyError('Cannot find pre-trained source {} for token embedding {}. '
                           'Valid pre-trained file names for embedding {}: {}'.format(
                               source, embedding_name, embedding_name,
                               ', '.join(source_file_hash.keys())))