private function addExpansionFields($fieldset, $model)
    {
        $form = $fieldset->getForm();

        /* @var $bagRenderer \Smile\ElasticsuiteThesaurus\Block\Adminhtml\Thesaurus\Renderer\Expansions */
        $bagRenderer = $this->getLayout()->createBlock(
            'Smile\ElasticsuiteThesaurus\Block\Adminhtml\Thesaurus\Renderer\Expansions'
        )->setForm($fieldset->getForm());

        $fieldset = $form->addFieldset('bag_of_words_fieldset', ['legend' => __('Bag of words')]);
        $fieldset->addField('bag_of_words', 'note', []);

        $form->getElement('bag_of_words_fieldset')
            ->setName('terms_relations')
            ->setValue($model->getTermsData())
            ->setRenderer($bagRenderer);
        $form->getElement('bag_of_words_fieldset')->getRenderer()->setValues($model->getTermsData());

        return $this;
    }