gistfile1.cs

public static IEnumerable<T> Filter<T>(this IEnumerable<T> items, Func<T, bool> pred)
{
    return items.Aggregate<T, Func<IEnumerable<T>, IEnumerable<T>>>(
        x => x, (f, c) => x => f(pred(c) ? (new [] { c }).Concat(x) : x))(Enumerable.Empty<T>());
}

public static IEnumerable<R> Map<T, R>(this IEnumerable<T> items, Func<T, R> proj)
{
    return items.Aggregate<T, Func<IEnumerable<R>, IEnumerable<R>>>(
        x => x, (f, c) => x => f((new[] { proj(c) }).Concat(x)))(Enumerable.Empty<R>());
}

public static IEnumerable<T> Flatten<T>(this IEnumerable<IEnumerable<T>> items)
{
    return items.Aggregate<IEnumerable<T>, Func<IEnumerable<T>, IEnumerable<T>>>(
        x => x, (f, c) => x => f(c.Concat(x)))(Enumerable.Empty<T>());
}

public static IEnumerable<T> Append<T>(this IEnumerable<T> source, IEnumerable<T> other)
{
    return source.Aggregate<T, Func<IEnumerable<T>,IEnumerable<T>>>(
        x => x, (f, c) => x => f((new [] { c }).Concat(x)))(other);
}