Program Instance elementsUnsized {A} {def : A} (l : list A) :
Unsized (elements def l).
Next Obligation.
rewrite ! semElementsSize. by case: l.
Qed.
(* A rather long frequency proof, probably we can do better *)

Lemma pick_imset A (def : G A) l :
pick def l @: [set m | m < sum_fst l] <--> [seq x <- l | x.1 != 0].