import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.CopyOnWriteArraySet;
import javax.annotation.CheckForNull;

/**
 * Registry of subscribers to a single event bus.
 *
 * @author Colin Decker
 */
@ElementTypesAreNonnullByDefault
final class SubscriberRegistry {

  /**
   * All registered subscribers, indexed by event type.
   *