docs: public static byte[] append(byte[] first, byte[] second) {
          if ((first == null) || (second == null)) return null;
          byte[] output = new byte[first.length + second.length];
          System.arraycopy(first, 0, output, 0, first.length);
          System.arraycopy(second, 0, output, first.length, second.length);
          return output;
      }