using System;

using System.Collections.Generic;

using System.Linq;

using static System.Console;

public class Program

{

    public static void Main()

    {

        WriteLine(Fibonacci(10));

        var range = Enumerable.Range(1, 10);

        var evens = Filter(range, (x) => x % 2 == 0);

        WriteLine(String.Join("; ", evens));

    }

    public static int Fibonacci(int x)

    {

        if (x < 0) throw new ArgumentException("Less negativity please!", nameof(x));

            return Fib(x).current;

​

        //Here is a local function

        (int current, int previous) Fib(int i)

        {

            if (i == 0) return (1, 0);

            var (p, pp) = Fib(i - 1);

            return (p + pp, p);

        }

    }

    public static IEnumerable<T> Filter<T>(IEnumerable<T> source, Func<T, bool> filter)

    {

        if (source == null) throw new ArgumentNullException(nameof(source));

        if (filter == null) throw new ArgumentNullException(nameof(filter));

​

        return Iterator();

​

        //Local function perfect for this purpose as it does not start running until MoveNext() is called on the IEnumerable

        IEnumerable<T> Iterator()

        {

            foreach (var element in source) 

            {

                if (filter(element)) { yield return element; }

            }

        }

    }

}