/// <summary>
/// Adds a <see cref="Versioned{T}"/> entry from <paramref name="entry"/>, identified by <paramref name="timestamp"/>,
/// and returns an updated <see cref="ORSet{T}"/>.
/// </summary>
public ORSet <T> Add(T entry, VectorTime timestamp) =>
new ORSet <T>(Versioned.Add(new Versioned <T>(entry, timestamp)));