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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 incrby(self, key, increment):
 """Increments the number stored at key by increment. If the key does not exist, it is set to 0 before performing the operation. An error is returned if the key contains a value of the wrong type or contains a string that can not be represented as integer. This operation is limited to 64 bit signed integers. See :meth:`~tredis.RedisClient.incr` for extra information on increment/decrement operations. .. versionadded:: 0.2.0 .. note:: **Time complexity**: ``O(1)`` :param key: The key to increment :type key: :class:`str`, :class:`bytes` :param int increment: The amount to increment by :returns: The value of key after the increment :rtype: int :raises: :exc:`~tredis.exceptions.RedisError` """

        return self._execute([b'INCRBY', key, ascii(increment)])