code: public int add(byte[] value, int valueOffset, int valueLength) {
      grow(length + valueLength);
      data.setBytes(length, value, valueOffset, valueLength);
      int result = length;
      length += valueLength;
      return result;
    }
docs: Copy a slice of a byte array into our buffer.
  @param value the array to copy from
  @param valueOffset the first location to copy from value
  @param valueLength the number of bytes to copy from value
  @return the offset of the start of the value