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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 reference_index(self):
 """Absolute path to the BWA index for EricScript reference data."""

        if self._db_location:
            ref_indices = glob.glob(os.path.join(self._db_location, "*", self._REF_INDEX))
            if ref_indices:
                return ref_indices[0]