How to use the pymoca.tree.flatten function in pymoca

def generate(ast_tree: ast.Tree, model_name: str):
    """
    :param ast_tree: AST to generate from
    :param model_name: class to generate
    :return: sympy source code for model
    """
    component_ref = ast.ComponentRef.from_string(model_name)
    ast_tree_new = copy.deepcopy(ast_tree)
    ast_walker = TreeWalker()
    flat_tree = flatten(ast_tree_new, component_ref)
    sympy_gen = SympyGenerator()
    ast_walker.walk(sympy_gen, flat_tree)
    return sympy_gen.src[flat_tree]

def generate(ast_tree: ast.Tree, model_name: str):
    """
    :param ast_tree: AST to generate from
    :param model_name: class to generate
    :return: sympy source code for model
    """
    component_ref = ast.ComponentRef.from_string(model_name)
    ast_tree_new = copy.deepcopy(ast_tree)
    ast_walker = TreeWalker()
    flat_tree = flatten(ast_tree_new, component_ref)
    gen = XmlGenerator()
    ast_walker.walk(gen, flat_tree)
    return etree.tostring(gen.xml[flat_tree], pretty_print=True).decode('utf-8')

def generate(ast_tree: ast.Tree, model_name: str, options: Dict[str, bool]=None) -> Model:
    """
    :param ast_tree: AST to generate from
    :param model_name: class to generate
    :param options: dictionary of generator options
    :return: casadi model
    """
    options = _merge_default_options(options)


    component_ref = ast.ComponentRef.from_string(model_name)
    ast_walker = GeneratorWalker()
    flat_tree = flatten(ast_tree, component_ref)
    component_ref_tuple = component_ref.to_tuple()
    casadi_gen = Generator(flat_tree, component_ref_tuple[-1], options)
    ast_walker.walk(casadi_gen, flat_tree)
    return casadi_gen.model