public static IAsyncEnumerable <TResult> SelectMany <TSource, TCollection, TResult>(this IAsyncEnumerable <TSource> source, Func <TSource, int, IAsyncEnumerable <TCollection> > collectionSelector, Func <TSource, TCollection, TResult> resultSelector) => LinqEnumerable.SelectMany(source, collectionSelector, resultSelector);
public static IAsyncEnumerable <TResult> SelectMany <TSource, TResult>(this IAsyncEnumerable <TSource> source, Func <TSource, int, IAsyncEnumerable <TResult> > selector) => LinqEnumerable.SelectMany(source, selector);