code: def exons(self, contig=None, strand=None):
          
          # DataFrame with single column called "exon_id"
          exon_ids = self.exon_ids(contig=contig, strand=strand)
          return [
              self.exon_by_id(exon_id)
              for exon_id in exon_ids
          ]
docs: Create exon object for all exons in the database, optionally
          restrict to a particular chromosome using the `contig` argument.