public static async ValueTask <V> GroupJoin <T, U, K, V>(this ValueTask <T> source,
ValueTask <U> inner,
Func <T, K> outerKeyMap,
Func <U, K> innerKeyMap,
Func <T, ValueTask <U>, V> project)
{
T t = await source.ConfigureAwait(false);
return(project(t, inner.Where(u => default(EqDefault <K>).Equals(outerKeyMap(t), innerKeyMap(u)))));
}
public static ValueTask <T> Filter <T>(this ValueTask <T> self, Func <T, bool> pred) => self.Where(pred);