The number of txns from the beginning of `uncommittedTxns` to commit
        :param count:
        :return: a tuple of 2 seqNos indicating the start and end of sequence
        numbers of the committed txns

def commitTxns(self, count: int) -> Tuple[Tuple[int, int], List]:
        """
        The number of txns from the beginning of `uncommittedTxns` to commit
        :param count:
        :return: a tuple of 2 seqNos indicating the start and end of sequence
        numbers of the committed txns
        """
        committedSize = self.size
        committedTxns = []
        for txn in self.uncommittedTxns[:count]:
            txn.update(self.append(txn))
            committedTxns.append(txn)
        self.uncommittedTxns = self.uncommittedTxns[count:]
        logger.debug('Committed {} txns, {} are uncommitted'.
                     format(len(committedTxns), len(self.uncommittedTxns)))
        if not self.uncommittedTxns:
            self.uncommittedTree = None
            self.uncommittedRootHash = None
        # Do not change `uncommittedTree` or `uncommittedRootHash`
        # if there are any `uncommittedTxns` since the ledger still has a
        # valid uncommittedTree and a valid root hash which are
        # different from the committed ones
        if committedTxns:
            return (committedSize + 1, committedSize + count), committedTxns
        else:
            return (committedSize, committedSize), committedTxns