using System;

using System.Linq;

using System.Collections.Generic;

class Program

{

    public void Main()

    {

        Console.WriteLine(

            Enumerable.Range(0,100).Fold(0, x => x+1 ).ToString()

        );

        Console.WriteLine(

            Enumerable.Range(1,100).FoldBack(1, x => x + x*x ).ToString()

        );

    }

}

​

static class ext

{

    public static IEnumerable<T> Tail<T>(this IEnumerable<T> self)

    {

        return self.Skip(1);

    }

​

    public static T Head<T>(this IEnumerable<T> self)

    {

        return self.First();

    }

    public static U Fold<T, U>(this IEnumerable<T> self, U state, Func<T, U, U> func)

    {

        foreach (var item in self)

        {

            state = func(self.Head(), state);

        }

        return state;

    }

    public static U Fold<T, U>(this IEnumerable<T> self, U state, Func<U, U> func)

    {

        foreach (var item in self)

        {

            state = func(state);

        }

        return state;

    }

    public static U FoldBack<T, U>(this IEnumerable<T> self, U state, Func<T, U, U> func)

    {

        foreach (var item in self.Reverse())

        {

            state = func(self.Head(), state);

        }

        return state;

    }

    public static U FoldBack<T, U>(this IEnumerable<T> self, U state, Func<U, U> func)

    {

        foreach (var item in self.Reverse())

        {

            state = func(state);

        }

        return state;

    }

}

​