function getSingle($type_uri, $default=null)
    {
        $values = Auth_OpenID::arrayGet($this->data, $type_uri);
        if (!$values) {
            return $default;
        } else if (count($values) == 1) {
            return $values[0];
        } else {
            return new Auth_OpenID_AX_Error(
              sprintf('More than one value present for %s',
                      $type_uri)
              );
        }
    }