The third line of
王之涣's
凉州词, answered by
春风不度玉门关 'the spring wind never crosses the Yumen Pass'.
杨柳 is the tune
折杨柳 'Breaking the Willow', a song of parting: people gave a willow twig to someone setting out, and beyond the pass, where spring never turns the willows green, the tune only deepens the longing.
