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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_import_source():
 """Check if tlgu imported, if not import it."""

        path_rel = '~/cltk_data/greek/software/greek_software_tlgu/tlgu.h'
        path = os.path.expanduser(path_rel)
        if not os.path.isfile(path):
            try:
                corpus_importer = CorpusImporter('greek')
                corpus_importer.import_corpus('greek_software_tlgu')
            except Exception as exc:
                logger.error('Failed to import TLGU: %s', exc)
                raise