Sentence:  He had yet to meet Harold Arlen , for although they had `` collaborated '' on `` Satan 's Li ' l Lamb '' , Mercer and Harburg had worked from a <WSD>lead</WSD> sheet the composer had furnished them . 
Meaning of the word marked with <WSD> tags:  a sheet containing the words and melody for a song (and some indication of harmony) written in simple form