private static Arbitrary <To> MapArb <To, From>(Arbitrary <From> arb, Func <From, To> func)
where To : class =>
Arb.Convert <From, To>(
FsFunc.Create <From, To>(a => {
var r = func(a);
convertBackTable.GetValue(r, _ => a);
return(r);
}),
FsFunc.Create <To, From>(b => convertBackTable.TryGetValue(b, out var result)