code: byte[][] rawKeys(K key, Collection<K> keys) {
          final byte[][] rawKeys = new byte[keys.size() + (key != null ? 1 : 0)][];
  
          int i = 0;
  
          if (key != null) {
              rawKeys[i++] = rawKey(key);
          }
  
          for (K k : keys) {
              rawKeys[i++] = rawKey(k);
          }
  
          return rawKeys;
      }
docs: Raw keys.
  
  @param key  the key
  @param keys the keys
  @return the byte[][]