namespace System.Collections.Frozen
{
    [DebuggerDisplay("Count = {Count}")]
    [DebuggerTypeProxy(typeof(ImmutableEnumerableDebuggerProxy<?>))]
    public abstract class FrozenSet<T> : ICollection<T>, IEnumerable<T>, IReadOnlyCollection<T>, ISet<T>, ICollection, IEnumerable
    {
        public bool IsSubsetOf(IEnumerable<T>! other);
    }
}