namespace System.Collections.Frozen
{
    [CollectionBuilder(typeof(FrozenSet), "Create")]
    public abstract class FrozenSet<T> : ICollection<T>, IEnumerable, IEnumerable<T>, IReadOnlyCollection<T>, IReadOnlySet<T>, ISet<T>, ICollection
    {
        public bool IsProperSupersetOf(IEnumerable<T>! other);
    }
}