问无法解析symbol EnumerableEx

EN

public static class Extensions
{
    public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> xs)
    {
        if (!xs.Any())
            return EnumerableEx.Return(Enumerable.Empty<T>());
        else
            return from zs in Permutations(xs.Skip(1))
                   from i in Enumerable.Range(0, zs.Count() + 1)
                   select zs.Take(i).Concat(EnumerableEx.Return(xs.First())).Concat(zs.Skip(i));
    }
}