code: def _add_gene_associations(self, r_id, r_genes, gene_ids, r_tag):
          
          genes = ET.SubElement(
              r_tag, _tag('geneProductAssociation', FBC_V2))
          if isinstance(r_genes, list):
              e = Expression(And(*(Variable(i) for i in r_genes)))
          else:
              e = Expression(r_genes)
          gene_stack = [(e.root, genes)]
          while len(gene_stack) > 0:
              current, parent = gene_stack.pop()
              if isinstance(current, Or):
                  gene_tag = ET.SubElement(parent, _tag('or', FBC_V2))
              elif isinstance(current, And):
                  gene_tag = ET.SubElement(parent, _tag('and', FBC_V2))
              elif isinstance(current, Variable):
                  gene_tag = ET.SubElement(parent, _tag(
                      'geneProductRef', FBC_V2))
                  if current.symbol not in gene_ids:
                      id = 'g_' + util.create_unique_id(
                          self._make_safe_id(current.symbol), gene_ids)
                      gene_ids[id] = current.symbol
                      gene_tag.set(_tag('geneProduct', FBC_V2), id)
              if isinstance(current, (Or, And)):
                  for item in current:
                      gene_stack.append((item, gene_tag))
docs: Adds all the different kinds of genes into a list.