ydb/yql/essentials/tests/s-expressions/suites/Optimizers/FoldOneItemStruct.yqls

(
(let config (DataSource 'config))

(let list (AsList (String 'a) (String 'b) (String 'c)))
(let data (Fold list (AddMember (Struct) 'z (String '"")) 
   (lambda '(item state) 
       (AddMember 
          (Struct) 
          'z 
          (Concat (Member state 'z) item)))))
(let res_sink (DataSink 'result))
(let world (Write! world res_sink
    (Key)
    data '('('type))))

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