docs: public static <E> Interner<E> newStrongInterner() {
      final ConcurrentMap<E, E> map = new MapMaker().makeMap();
      return new Interner<E>() {
        @Override
        public E intern(E sample) {
          E canonical = map.putIfAbsent(checkNotNull(sample), sample);
          return (canonical == null) ? sample : canonical;
        }
      };
    }