Not sure how to populate the IEnumerable 'result'

Jacket.cs

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

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

        public static IEnumerable<T3> Zipper<T1, T2, T3>(this IEnumerable<T1> first, IEnumerable<T2> second, Func<T1, T2, T3> f)
        {
            IEnumerable<T3> result;
            var FWalk = first.GetEnumerator();
            var SWalk = second.GetEnumerator();



            foreach (var item in first)
            {
                FWalk.MoveNext();
                SWalk.MoveNext();

                f(FWalk.Current, SWalk.Current);

            }




            return result;
        }



    }