June Tabor recorded Les Barker's parody, TRAINS OF WATERLOO , which is also in the Digital Tradition.

But there's no tuning for that - just June and Lesley Davies singing alternate verses solo and unaccompanied. (Lesley the odd ones and June the even ones)

Is there a single word for this? Can't call it "a capella", because there's only ever one of them singing at any time. Nor a duet, for the same reason.