code: def add_ret_hash_memo(self, memo_return):
          
          memo_return = memo.RetHashMemo(memo_return)
          return self.add_memo(memo_return)
docs: Set the memo for the transaction to a new :class:`RetHashMemo
          <stellar_base.memo.RetHashMemo>`.
  
          :param bytes memo_return: A 32 byte hash or hex encoded string intended to be interpreted as
              the hash of the transaction the sender is refunding.
          :type memo_return: bytes, str
          :return: This builder instance.