public static IAsyncEnumerable <TResult> SelectMany <TSource, TResult>(this Task <IEnumerable <TSource> > source, Func <TSource, IAsyncEnumerable <TResult> > selector)
=> selector == null ? throw new ArgumentNullException(nameof(selector))
: AsyncIteratorEnumerable.Create(() => new SelectManyIterator <TSource, TResult, TResult>(source.AsAsyncEnumerable(), (o, _) => selector(o), (_, o) => o));
public static IAsyncEnumerable <TSource> Concat <TSource>(this IAsyncEnumerable <TSource> first, Task <IEnumerable <TSource> > second) => AsyncIteratorEnumerable.Create(() => new ConcatIterator <TSource>(first, second.AsAsyncEnumerable()));
