docs: @Nonnull
      public static <T> List<T> join(@Nonnull Collection<? extends T>... items) {
          int size = 0;
          for (Collection<? extends T> item : items)
              size += item.size();
          List<T> r = new ArrayList<>(size);
          for (Collection<? extends T> item : items)
              r.addAll(item);
          return r;
      }