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 pickDrop_def :
forall {A} (l: list (nat * G (option A))) n,
sum_fst l <= n ->
pickDrop l n = (0, returnGen None, l).