public static Task <TResult> SelectOptionalAsync <TItem, TSelect, TCarry1, TCarry2, TResult>(this IEnumerableAsync <TItem> enumerable,
TCarry1 carry1, TCarry2 carry2,
Func <TItem, TCarry1, TCarry2,
Func <TSelect, TCarry1, TCarry2, ISelectionResult <TSelect> >,
Func <TCarry1, TCarry2, ISelectionResult <TSelect> >,
Task <ISelectionResult <TSelect> > > selectionAsync,
Func <TSelect[], TCarry1, TCarry2, TResult> aggregation)
{
return(enumerable.SelectOptionalAsync(carry1.PairWithValue(carry2),
(item, carryUpdated, next, skip) => selectionAsync(item, carryUpdated.Key, carryUpdated.Value,
(selected, carry1Updated, carry2Updated) => next(selected, carry1Updated.PairWithValue(carry2Updated)),
(carry1Updated, carry2Updated) => skip(carry1Updated.PairWithValue(carry2Updated))),
(TSelect[] selectedItems, KeyValuePair <TCarry1, TCarry2> carryFinal) =>
aggregation(selectedItems, carryFinal.Key, carryFinal.Value)));
}
