Sentence:  I left Holmes seated in front of the smouldering fire, and long into the watches of the night I heard the low, melancholy wailings of his violin, and knew that he was still pondering over the strange problem which he had set himself to <WSD>unravel</WSD>. 
Meaning of the word marked with <WSD> tags:  To clear from complication or difficulty; to unfold; to solve.