One alternative would be
XX author
unknow value with the disputer as a source.
To express uncertainty we could also use a statement which says the author is
one of <the french painter in the years 1500 minus Leonardo>, and create the appropriate class, although we do not have all the expressive power right now to say that. Basic set operation like "set union" or "set complement in another set"
or "disjoint with" could be good for that by the way (unfortunaltely disjoint with has not really been well accepted by community).