C# 7 Local Function example with tuples in a tail call

Program.cs

using System;

// Visual Studio 15 Preview 3
// Conditional compilation symbols:  __DEMO__ ,__DEMO_EXPERIMENTAL__
// Manually added ValueTuples.cs to project https://raw.githubusercontent.com/dotnet/roslyn/future/docs/features/ValueTuples.cs
namespace TupleExamples
{
    class Program
    {
        static void Main(string[] args)
        {
            long Factorial(int x)
            {
                long Aux((int acc, int n) _)
                {
                    if (_.n <= 1) return _.acc;
                    return Aux((_.acc * _.n, _.n - 1));
                }

                return Aux((1, x));
            }

            Console.WriteLine($"1! = {Factorial(1)}");
            Console.WriteLine($"5! = {Factorial(5)}");
            Console.WriteLine($"20! = {Factorial(20)}");

            Console.ReadKey();
        }
    }
}