namespace System.Collections.Generic;
/// Items each there once, compared as sets, read only.
[Intrinsic]
public interface IReadOnlySet<T> : IReadOnlyCollection<T> {
readonly bool Contains(T item);
readonly bool IsSubsetOf(in IEnumerable<T> other);
readonly bool IsSupersetOf(in IEnumerable<T> other);
/// A subset of `other` that isn't all of it.
readonly bool IsProperSubsetOf(in IEnumerable<T> other);
readonly bool IsProperSupersetOf(in IEnumerable<T> other);
readonly bool Overlaps(in IEnumerable<T> other);
readonly bool SetEquals(in IEnumerable<T> other);
}