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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 _has_sj_index(ref_file):
 """this file won't exist if we can do on the fly splice junction indexing"""

    return (file_exists(os.path.join(ref_file, "sjdbInfo.txt")) and
            (file_exists(os.path.join(ref_file, "transcriptInfo.tab"))))