public static function index_by_first_val(array $ar): array
  {
    if ( empty($ar) || !isset($ar[0]) || !\count($ar[0]) ){
      return $ar;
    }
    $cols = array_keys($ar[0]);
    $idx = array_shift($cols);
    $num_cols = \count($cols);
    $res = [];
    foreach ( $ar as $d ){
      $index = $d[$idx];
      unset($d[$idx]);
      $res[$index] = $num_cols > 1 ? $d : $d[$cols[0]];
    }
    return $res;
  }