num.cs

using System;

static class Program {

    static void Main() {
        Zero().Succ().Succ().Pred().Succ().Pred().Succ().Succ().Show();

        var three = Zero().Succ().Succ().Succ();
        var four = Zero().Succ().Succ().Succ().Succ();

        // How to write??????
        // Plus(three, four).Show();
        // Twice(three).Show();
        // Mult(three, four).Show()l
    }

    static Z Zero() => new Z();
    static S<N> Succ<N>(this N n) where N : Nat => new S<N>();
    static N Pred<N>(this S<N> n) where N : Nat, new() => new N();
}

abstract class Nat {
    public void Show() => Console.WriteLine(this);
}
class Z : Nat {}
class S<T> : Nat where T : Nat {}