code: def species_string(self):
          
          if self.is_ordered:
              return list(self.species.keys())[0].__str__()
          else:
              sorted_species = sorted(self.species.keys())
              return ", ".join(["{}:{:.3f}".format(sp, self.species[sp])
                                for sp in sorted_species])
docs: String representation of species on the site.