code: def add(self, conn, key, value, exptime=0):
          
          flags = 0  # TODO: fix when exception removed
          return (yield from self._storage_command(
              conn, b'add', key, value, flags, exptime))
docs: Store this data, but only if the server *doesn't* already
          hold data for this key.
  
          :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.