<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def prepend(self, conn, key, value, exptime=0):
 """Add data to an existing key before existing data :param key: ``bytes``, is the key of the item. :param value: ``bytes``, data to store. :param exptime: ``int`` is expiration time. If it's 0, the item never expires. :return: ``bool``, True in case of success. """

        flags = 0  # TODO: fix when exception removed
        return (yield from self._storage_command(
            conn, b'prepend', key, value, flags, exptime))