(
(let config (DataSource 'config))

(let list (Just (String 'a)))
(let data (Fold1 list 
(lambda '(item) (Concat item (String 'b)))
(lambda '(item state) (Concat item state))))
(let res_sink (DataSink 'result))
(let world (Write! world res_sink
    (Key)
    data '('('type))))

(let world (Commit! world res_sink))
(return world)
)