public static IAsyncEnumerable <TSource> Union <TSource>(this IAsyncEnumerable <TSource> first, IAsyncEnumerable <TSource> second, IEqualityComparer <TSource> comparer) => LinqEnumerable.Union(first, second, comparer);
public static IAsyncEnumerable <TSource> Union <TSource>(this IAsyncEnumerable <TSource> first, IAsyncEnumerable <TSource> second) => LinqEnumerable.Union(first, second);