; Iter it.2 positive (spec D1): foldl-shape accumulator. `go` recurses
; on the Cons-tail `t` at position 0 (structurally guarded) while
; threading an unconstrained accumulator `acc` at position 1. The
; accumulator position is never examined by the guardedness check, so
; this classifies as structural recursion: pure, total, Diverge-free,
; with NO `tail-app` marker. This is the single most LLM-natural
; iteration shape and must stay structural (spec D1).

(module struct_rec_foldl_sum

  (data IntList
    (ctor INil)
    (ctor ICons (con Int) (con IntList)))

  (fn go
    (doc "Sum via a foldl-shape accumulator; structural on the tail.")
    (type
      (fn-type
        (params (con IntList) (con Int))
        (ret (con Int))))
    (params xs acc)
    (body
      (match xs
        (case (pat-ctor INil) acc)
        (case (pat-ctor ICons h t)
          (app go t (app + acc h)))))))