code: protected function addBaseFieldset()
      {
          if (null === $this->baseFieldset) {
              return;
          }
          
          $fs = $this->baseFieldset;
          if (!is_array($fs)) {
              $fs = array(
                  'type' => $fs,
              );
          }
          if (!isset($fs['options']['use_as_base_fieldset'])) {
              $fs['options']['use_as_base_fieldset'] = true;
          }
          $this->add($fs);
      }
docs: Adds the base fieldset.