public function cleanUp( $expires = 0 )
	{
		$expires	= $expires ? $expires : $this->expires;
		if( !$expires )
			throw new InvalidArgumentException( 'No expire time given or set on construction.' );

		$number	= 0;
		$index	= new DirectoryIterator( $this->path );
		foreach( $index as $entry )
		{
			if( $entry->isDot() || $entry->isDir() )
				continue;
			$pathName	= $entry->getPathname();
			if( substr( $pathName, -7 ) !== ".serial" )
				continue;
			if( $this->isExpired( $pathName, $expires ) )
				$number	+= (int) @unlink( $pathName );
		}
		return $number;
	}