Instruments on willow trees

I just found a beatiful picture in a psalm:

"By the rivers of Babylon--
there we sat down and there we wept
when we remembered Zion.
On the willows there
we hung up our harps.

For there our captors
asked us for songs,
and our tormentors asked for mirth, saying,
"Sing us one of the songs of Zion!"

How could we sing the Lord's song
in a foreign land?"

Hanging the instruments on the trees, that's so individually expressing image of sadness, homesickness and resistance all togheter. The end of the psalm is shockingly cruel. It is number 137.