code: def exon_intervals(self):
          
          results = self.db.query(
              select_column_names=["exon_number", "start", "end"],
              filter_column="transcript_id",
              filter_value=self.id,
              feature="exon")
          sorted_intervals = [None] * len(results)
          for (exon_number, start, end) in results:
              sorted_intervals[int(exon_number) - 1] = (start, end)
          return sorted_intervals
docs: List of (start,end) tuples for each exon of this transcript,
          in the order specified by the 'exon_number' column of the
          exon table.