docs: public static HashFunction concatenating(Iterable<HashFunction> hashFunctions) {
      checkNotNull(hashFunctions);
      // We can't use Iterables.toArray() here because there's no hash->collect dependency
      List<HashFunction> list = new ArrayList<HashFunction>();
      for (HashFunction hashFunction : hashFunctions) {
        list.add(hashFunction);
      }
      checkArgument(list.size() > 0, "number of hash functions (%s) must be > 0", list.size());
      return new ConcatenatedHashFunction(list.toArray(new HashFunction[0]));
    }