protected function minimizeResource(string $resource, ?string $fullPathName): string
  {
    // If $theFullPathName is not set $resource is concatenation of 2 or more optimized CSS file. There is no need to
    // convert relative paths and minimized $resource again. Moreover, it is not possible to convert relative paths
    // since $resource can be a concatenation of CSS files from different subdirectories.
    if (isset($fullPathName))
    {
      $css = $this->convertRelativePaths($resource, $fullPathName);

      list($std_out, $std_err) = $this->runProcess($this->minifyCommand, $css);

      if ($std_err) $this->logInfo($std_err);

      $ret = $std_out;
    }
    else
    {
      $ret = $resource;
    }

    return $ret;
  }