private function getChangeExtent($fromLine, $toLine)
	{
		$start = 0;
		$limit = min(strlen($fromLine), strlen($toLine));
		while($start < $limit && $fromLine{$start} == $toLine{$start}) {
			++$start;
		}
		$end = -1;
		$limit = $limit - $start;
		while(-$end <= $limit && substr($fromLine, $end, 1) == substr($toLine, $end, 1)) {
			--$end;
		}
		return array(
			$start,
			$end + 1
		);
	}