docs: public Set<DeviceAnnouncement> getCurrentDevices() {
          if (!isRunning()) {
              throw new IllegalStateException("DeviceFinder is not active");
          }
          expireDevices();  // Get rid of anything past its sell-by date.
          // Make a copy so callers get an immutable snapshot of the current state.
          return Collections.unmodifiableSet(new HashSet<DeviceAnnouncement>(devices.values()));
      }