Wrap an object into an IEnumerable of single item

Code:

public static IEnumerable<T> Yield<T>(this T item)
{
    yield return item;
}

Faster TakeBy

Code:

public static IEnumerable<IEnumerable<T>> TakeBy<T>(this IEnumerable<T> source, int count)
{
    var grps = source.Select((val, i) => new { idx = i / count, val }).GroupBy(e => e.idx);
    return grps.Select(g => g.Select(e => e.val));
}

Combinatorics: get all combinations of k elements from an input sequence

Code:

public static IEnumerable<IEnumerable<T>> Combine<T>(this IEnumerable<T> list, int k = 2, bool repetition = false)
{
    for (int i = 0; i < list.Count(); i++)
    {
        T cur = list.Skip(i).First();
        if (k == 1)
        {
            yield return cur.Yield();
        }
        else
        {
            foreach (var res in list.Skip(i + (repetition ? 0 : 1)).Combine(k - 1, repetition))
            {
                yield return res.Prepend(cur);
            }
        }
    }
}

e.g.
{ 1, 2, 3 }.Combine(2) = {{ 1, 2 }, { 1, 3 }, { 2, 3 }}
{ 1, 2 }.Combine(2, true) = {{ 1, 1 }, { 1, 2 }, { 2, 2 }}
{ 1, 2, 3, 4 }.Combine(3) = {{ 1, 2, 3 }, { 1, 2, 4 }, { 2, 3, 4 }}