public function startGroup($options = []) {
        $options += [
            'id'                   => 'panelGroup-'.(++$this->_groupCount),
            'collapsible'          => true,
            'open'                 => 0,
            'templateVars' => []
        ];
        $this->_states->push('group', [
            'groupPanelOpen' => $options['open'],
            'groupPanelCount' => -1,
            'groupId' => $options['id'],
            'groupCollapsible' => $options['collapsible']
        ]);
        return $this->formatTemplate('panelGroupStart', [
            'attrs' => $this->templater()->formatAttributes($options, ['open', 'collapsible']),
            'templateVars' => $options['templateVars']
        ]);
    }