namespace System.Collections.Immutable
{
    [DebuggerDisplay("Count = {Count}")]
    [DebuggerTypeProxy(typeof(ImmutableEnumerableDebuggerProxy<?>))]
    [DefaultMember("Item")]
    public sealed class ImmutableSortedSet<T> : ICollection<T>, IEnumerable<T>, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T>, ISet<T>, ICollection, IEnumerable, IList, IImmutableSet<T>
    {
        [DebuggerDisplay("Count = {Count}")]
        [DebuggerTypeProxy(typeof(ImmutableSortedSetBuilderDebuggerProxy<?>))]
        [DefaultMember("Item")]
        public sealed class Builder : ICollection<T>, IEnumerable<T>, IReadOnlyCollection<T>, ISet<T>, ICollection, IEnumerable
        {
            public bool IsProperSubsetOf(IEnumerable<T>! other);
        }
    }
}