Schwartz Stephen - On The Willows lyrics

(Psalms 137:1-4) On the willows, there We hung up our lyres For our captors there Require Of us songs And our tormentors mirth On the willows, there We hung up our lyres For our captors there Require Of us songs And our tormentor's mirth Saying: "Sing us one Of the songs of Zion Sing us one Of the songs of Zion Sing us one Of the songs of Zion" But how can we sing? Sing the Lord's songs? In a foreign land?