code: protected static function getReader(?string $identifier): ReaderInterface
      {
          if (!isset(static::$identifiers[$identifier])) {
              throw new \RuntimeException('Unknown identifier "' . $identifier . '" for a configuration file.');
          }
  
          $readerClass = static::$identifiers[$identifier];
  
          return new $readerClass();
      }
docs: Returns a reader for the provided $identifier.
  
  @param string|null $identifier
  
  @throws \RuntimeException if the provided $identifier is not valid
  
  @return ReaderInterface