I don't know what a folk song is and I have ceased to care. A C# song
is a non-degenerate equivalence class of tunes and words at least one member of which was collected by Cecil Sharp, or would have been collected by him had he come across it. The equivalence relation, which is perhaps not well defined, is that any reasonable person upon hearing two members of the class would say, "the words are different and the tunes are different, but they are obviously the same song.