System.Collections.Immutable.ImmutableHashSet<T>
public sealed class ImmutableHashSet<T> : IImmutableSet<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable, IHashKeyCollection<T>, ICollection<T>, ISet<T>, ICollection, IStrongEnumerable<T, Enumerator<T>>
public sealed class Builder<T> : IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable, ISet<T>, ICollection<T>