namespace System.Collections.Generic
{
    public class HashSet<T> : ICollection<T>, IEnumerable, IEnumerable<T>, IReadOnlyCollection<T>, IReadOnlySet<T>, ISet<T>, IDeserializationCallback, ISerializable
    {
        public readonly struct AlternateLookup<TAlternate>
        {
            public bool Add(TAlternate item);
        }
    }
}