Keep
Signature
keep: {
<T>(kept: T): Keep_F<T, T>;
of: <Kept, Value>(value: Value, keep: Keep_F<Kept, Value>) => Keep_F<Kept, Value>;
map: <Kept, Prev, New>(fn: (value: Prev, kept: Kept) => New, keep: Keep_F<Kept, Prev>) => Keep_F<Kept, New>;
value: <Kept, Value>(keep: Keep_F<Kept, Value>) => Value;
}