Adds all the different kinds of genes into a list.

def _add_gene_associations(self, r_id, r_genes, gene_ids, r_tag):
        """Adds all the different kinds of genes into a list."""
        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))