;; The hard case, answered without looking at a single call site: a function
;; value that arrives as a parameter may carry an environment on its caller's
;; frame, so a function that *stores* one is refused where it is written.
;;
;; That is what makes passing a capturing fn down safe everywhere — no callee
;; can keep it — and it is also the conservative half: this particular [keep]
;; would be harmless for a caller that passed a name, and there is no way for
;; the definition to know that it did.
(defn keep [f (Fn [] i32)] (Fn [] i32) f)

(defn main [] i32 (println ((keep (fn [] 1)))) 0)