code: def bed12(self, score="0", rgb="."):
          
          if not self.is_gene_pred:
              raise CruzException("can't create bed12 from non genepred feature")
          exons = list(self.exons)
          # go from global start, stop, to relative start, length...
          sizes = ",".join([str(e[1] - e[0]) for e in exons]) + ","
          starts = ",".join([str(e[0] - self.txStart) for e in exons]) + ","
          name = self.name2 + "," + self.name if hasattr(self, "name2") \
                                              else self.name
          return "\t".join(map(str, (
              self.chrom, self.txStart, self.txEnd, name,
              score, self.strand, self.cdsStart, self.cdsEnd, rgb,
              len(exons), sizes, starts)))
docs: return a bed12 (http://genome.ucsc.edu/FAQ/FAQformat.html#format1)
          representation of this interval