诗有咏正面难于出色，而侧击旁敲，更为得力者，此类诗是也。苏武，绝域羁臣，备尝艰苦。作者既咏悲笳感人，复借笳声以咏苏武，用“一听头白”四字，以见十九年中历人所难堪之境；况悠长岁月，所闻者宁止胡笳！此二句，所谓力透纸背矣。

《边上闻胡笳》 杜牧

何处吹笳薄暮天？塞垣高鸟没狼烟。
游人一听头堪白，苏武曾经十九年。

Hearing Tartar Clarinet on the Border
Du Mu

Whence is blown the clarinet at sunset?
Border mountain parapets 'neath a high bird
Hideth wolf dung smoke columns from one's sight.
Visitors hearing it would have their hair turned white.
For nineteen years all along, Su Wu had this heard!