def add(self, key, value, expire=0, noreply=None):
        """
        The memcached "add" command.

        Args:
          key: str, see class docs for details.
          value: str, see class docs for details.
          expire: optional int, number of seconds until the item is expired
                  from the cache, or zero for no expiry (the default).
          noreply: optional bool, True to not wait for the reply (defaults to
                   self.default_noreply).

        Returns:
          If noreply is True, the return value is always True. Otherwise the
          return value is True if the value was stored, and False if it was
          not (because the key already existed).
        """
        if noreply is None:
            noreply = self.default_noreply
        return self._store_cmd(b'add', {key: value}, expire, noreply)[key]