/**
  * Updates the global wp_rewrite instance if it is wrong
  *
  * @param  string $taxonomy
  * @return bool|string FALSE or the original string for later restore
  */
 private function fix_term_base($taxonomy)
 {
     $expected = $this->get_expected_base($taxonomy);
     $existing = $this->wp_rewrite->get_extra_permastruct($taxonomy);
     if (!$this->update_required($expected, $existing)) {
         return FALSE;
     }
     $this->set_permastruct($taxonomy, $expected);
     return $existing;
 }