namespace System.Linq;
/// Items by their keys, many to a key (`ToLookup`).
[Intrinsic]
public interface ILookup<TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>> {
i32 Count { get; }
/// The items of `key`; none when it has none.
IEnumerable<TElement> this[TKey key] { get; }
bool Contains(TKey key);
}