code: def signature(self):
          
          sig_elems = [self.seqid, self.source, self.type, \
                      self.start, self.end, self.strand, \
                      self.phase]
          if re.search("exon|CDS|UTR", self.type):
              parent = self.get_attr("Parent")
              if parent:
                  (locus, iso) = atg_name(parent, retval="locus,iso", \
                          trimpad0=False)
                  if locus:
                      sig_elems.append(locus)
          else:
              sig_elems.extend([self.accn])
  
          return ",".join(str(elem) for elem in sig_elems)
docs: create a unique signature for any GFF line based on joining
          columns 1,2,3,4,5,7,8 (into a comma separated string)