Enumerable expansion, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int[] data = new int[5];
            Random rand = new Random();
            data = data.Fill(() => rand.Next()).ToArray();
            Console.WriteLine(data.ArrString());
            
            Console.WriteLine(data
                              .Map(i => BitConverter.GetBytes(i))
                              .ArrString()
                             );
            Console.WriteLine(data
                              .Map(i => BitConverter.GetBytes(i))
                              .Expand()
                              .Map(b => b.ToString("x2"))
                              .ArrString()
                             );
        }
    }
    static class Extn
    {
        public static IEnumerable<Out> Map<In,Out>(this IEnumerable<In> a, Func<In,Out> f)
        {
            foreach(In i in a) yield return f(i);
        }
        public static IEnumerable<T> Expand<T>(this IEnumerable<IEnumerable<T>> a)
        {
            foreach(IEnumerable<T> e in a)
                foreach(T t in e) yield return t;
        }
        public static Out Fold<In,Out>(this IEnumerable<In> a, Out initial, Func<In,Out,Out> f)
        {
            Out tor = initial;
            foreach(In i in a) tor = f(i, tor);
            return tor;
        }
        public static string ArrString<T>(this IEnumerable<T> a)
        {
            string res = a.Fold("[", (i, o) => o + i.ToString() + ", ");
            return res.Remove(res.Length - 2) + "]";
        }
        public static IEnumerable<T> Fill<T>(this IEnumerable<T> a, Func<T> f)
        {
            int L = a.Count();
            for(int x = 0; x < L; x++) yield return f();
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,17 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 14 Mb, average nr of threads: 3