showBuildGroupForm example

$this->showBuildGroupButton($form, $form_state);
    }
    $form['clear_markup_start'] = [
      '#markup' => '<div class="clearfix">',
    ];
    if ($this->isAGroup()) {
      if ($this->canBuildGroup()) {
        $form['clear_markup_start'] = [
          '#markup' => '<div class="clearfix">',
        ];
        // Render the build group form.         $this->showBuildGroupForm($form, $form_state);
        $form['clear_markup_end'] = [
          '#markup' => '</div>',
        ];
      }
    }
    else {
      // Add the subform from operatorForm().       $this->showOperatorForm($form, $form_state);
      // Add the subform from valueForm().       $this->showValueForm($form, $form_state);
      $form['clear_markup_end'] = [