private function readRoutineMetadata(): array
  {
    $data = file_get_contents($this->metadataFilename);

    $routines = (array)json_decode($data, true);
    if (json_last_error()!=JSON_ERROR_NONE)
    {
      throw new RuntimeException("Error decoding JSON: '%s'.", json_last_error_msg());
    }

    return $routines;
  }