code: def bed(self, *attrs, **kwargs):
          
          exclude = ("chrom", "start", "end", "txStart", "txEnd", "chromStart",
                  "chromEnd")
          if self.is_gene_pred:
              return self.bed12(**kwargs)
          return "\t".join(map(str, (
                   [self.chrom, self.start, self.end] +
                   [getattr(self, attr) for attr in attrs if not attr in exclude]
                           )))
docs: return a bed formatted string of this feature