Exception ExceptionOrDefault(Exceptional <T> ex) => ex.Match(exc => exc, _ => default(Exception));
bool IsSuccess(Exceptional <T> ex) => ex.Match(_ => false, _ => true);
T ValueOrDefault(Exceptional <T> ex) => ex.Match(exc => default(T), t => t);
public static Validation<Exceptional<R>> Traverse<T, R>
(this Exceptional<T> tr, Func<T, Validation<R>> f)
=> tr.Match(
Exception: e => Valid((Exceptional<R>)e),
Success: t => from r in f(t) select Exceptional(r));