Sentence:  The vocal <WSD>nocturne</WSD>, in other words, turns out to be no easier a genre to define than the piano <WSD>nocturne</WSD>. But its ambiguities are nevertheless helpful in making sense of those of the piano <WSD>nocturne</WSD>. 
Meaning of the word marked with <WSD> tags:  A dreamlike or pensive composition, usually for the piano.