docs: def _add_gene_list(self, parent_tag, gene_id_dict):
          """Create list of all gene products as sbml readable elements."""
          list_all_genes = ET.SubElement(parent_tag, _tag(
              'listOfGeneProducts', FBC_V2))
          for id, label in sorted(iteritems(gene_id_dict)):
              gene_tag = ET.SubElement(
                  list_all_genes, _tag('geneProduct', FBC_V2))
              gene_tag.set(_tag('id', FBC_V2), id)
              gene_tag.set(_tag('label', FBC_V2), label)