code: def get_bytes(self, index, size):
          
          pos = self.size() // 8 - 1 - index
          return self[pos * 8 + 7 : (pos - size + 1) * 8]
docs: Extracts several bytes from a bitvector, where the index refers to the byte in a big-endian order
  
          :param index: the byte index at which to start extracting
          :param size: the number of bytes to extract
          :return: A BV of size ``size * 8``