The Undo attempted was in JRiver MC itself.
OK, I didn't do a backup. ( I've just started using JRiver).
I wasn't expecting any harm from this action.
By the way, is there a backup feature, or does one have to read the manual and figure out how to do a backup?
Actually, what I was trying to do was experimental. And what I was ultimately trying to do is make a sort of reliable 'Record' feature I asked about earlier. But what and why I was trying to do this will be a bit of a diversion I think. But I'll tell you if you want to know. I had noticed that from iTunes, when I drag and drop the selections in a playlist, they appear in the playlist order in the destination folder. I had hoped to drag and drop these into some tool for joining audio files.
As I say, this was only an experiment. I did not have very high hopes of it's working, but it was worth a try. Anyway it failed, but the surprise is that my library is now corrupted.