protected function addDir($dir)
{
// Add any additional directories to the top of the array
// so that the user can override art
array_unshift($this->art_dirs, rtrim($dir, '/'));
// Keep the array clean
$this->art_dirs = array_unique($this->art_dirs);
$this->art_dirs = array_filter($this->art_dirs);
$this->art_dirs = array_values($this->art_dirs);
}