public static IAsyncEnumerable <TResult> Join <TOuter, TInner, TKey, TResult>(this IAsyncEnumerable <TOuter> outer, IAsyncEnumerable <TInner> inner, Func <TOuter, TKey> outerKeySelector, Func <TInner, TKey> innerKeySelector, Func <TOuter, TInner, TResult> resultSelector, IEqualityComparer <TKey> comparer) => LinqEnumerable.Join(outer, inner, outerKeySelector, innerKeySelector, resultSelector, comparer);