public function group($name, $balise, callable $callback, $attr = null)
    {
        $form  = new FormBuilder([]);
        call_user_func_array($callback, [ &$form ]);
        $group = $this->merge_attr([ 'balise' => $balise ], $attr);

        return $this->input($name, [ 'type' => 'group', 'subform' => $form, 'attr' => $group ]);
    }