* in {@linkplain WeakReference weak} references.
 *
 * <p>Usage example:
 *
 * <pre>{@code
 * ConcurrentMap<Request, Stopwatch> timers = new MapMaker()
 *     .concurrencyLevel(4)
 *     .weakKeys()
 *     .makeMap();
 * }</pre>
 *
 * <p>These features are all optional; {@code new MapMaker().makeMap()} returns a valid concurrent