'Append given data to the value of an existing key.
@param key: The key to modify.
@type key: L{bytes}
@param val: The value to append to the current value associated with
the key.
@type val: L{bytes}
@return: A deferred that will fire with C{True} if the operation has
succeeded, C{False} otherwise.
@rtype: L{Deferred}'

def append(self, key, val):

    return self._set('append', key, val, 0, 0, '')
