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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def count(self, txn, prefix=None):
 """ Count number of records in the persistent map. When no prefix is given, the total number of records is returned. When a prefix is given, only the number of records with keys that have this prefix are counted. :param txn: The transaction in which to run. :type txn: :class:`zlmdb.Transaction` :param prefix: The key prefix of records to count. :type prefix: object :returns: The number of records. :rtype: int """

        key_from = struct.pack('>H', self._slot)
        if prefix:
            key_from += self._serialize_key(prefix)
        kfl = len(key_from)

        cnt = 0
        cursor = txn._txn.cursor()
        has_more = cursor.set_range(key_from)
        while has_more:
            _key = cursor.key()
            _prefix = _key[:kfl]
            if _prefix != key_from:
                break
            cnt += 1
            has_more = cursor.next()

        return cnt