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public static class EnumerableExtensions
{
    public static IEnumerable<T> Flatten<T>(this T value, Func<T, T> inner, T tail = default(T))
    {
        var comparer = EqualityComparer<T>.Default;
        while (!comparer.Equals(value, tail))
        {
            yield return value;
            value = inner(value);
        }
    }

    public static IEnumerable<T> Flatten<T>(this T value, Func<T, IEnumerable<T>> inner)
    {
        foreach (var i in inner(value))
            foreach (var j in Flatten(i, inner))
                yield return j;
        yield return value;
    }

    public static IEnumerable<T> Each<T>(this IEnumerable<T> items, Action<T> action)
    {
        foreach (var i in items)
            action(i);
        return items;
    }

    public static IEnumerable<T> Each<T>(this IEnumerable<T> items, Action<T, int> action)
    {
        var index = 0;
        foreach (var i in items)
            action(i, index++);
        return items;
    }

    public static IEnumerable<T> Prepend<T>(this IEnumerable<T> items, T item)
    {
        yield return item;
        foreach (var i in items)
            yield return i;
    }

    public static IEnumerable<T> Append<T>(this IEnumerable<T> items, T item)
    {
        foreach (var i in items)
            yield return i;
        yield return item;
    }
}