Append and Prepend in IEnumerable object

/// <summary>Adds a single element to the end of an IEnumerable.</summary>
        /// <typeparam name=”T”>Type of enumerable to return.</typeparam>
        /// <returns>IEnumerable containing all the input elements, followed by the
        /// specified additional element.</returns>
        public static IEnumerable<T> Append<T>(this IEnumerable<T> source, T element)
        {
            if (source == null)
                throw new ArgumentNullException(“source”);
            return concatIterator(element, source, false);
        }

        /// <summary>Adds a single element to the start of an IEnumerable.</summary>
        /// <typeparam name=”T”>Type of enumerable to return.</typeparam>
        /// <returns>IEnumerable containing the specified additional element, followed by
        /// all the input elements.</returns>
        public static IEnumerable<T> Prepend<T>(this IEnumerable<T> tail, T head)
        {
            if (tail == null)
                throw new ArgumentNullException(“tail”);
            return concatIterator(head, tail, true);
        }

        private static IEnumerable<T> concatIterator<T>(T extraElement,
            IEnumerable<T> source, bool insertAtStart)
        {
            if (insertAtStart)
                yield return extraElement;
            foreach (var e in source)
                yield return e;
            if (!insertAtStart)
                yield return extraElement;
        }