This compiles, and it makes perfect sense from a technical standpoint. But I don't know how I figured it out. And that bothers me.

SelectSoMany.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SelectSoMany
{
    public static class Program
    {
        static void Main(string[] args)
        {
        }

        public static IEnumerable<T2> SelectTooMany<T, T2>(this IEnumerable<T> xs, Func<T, IEnumerable<T2>> f)
        {


            var result = new List<T2>();
            var coll = xs.GetEnumerator();

            while (coll.MoveNext())
            {
                var current = f(coll.Current);
                foreach (var item in current)
                {
                    result.Add(item);
                }
            }


            return result;
        }






    }
}