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 sum_fstE A x (a : A) l:
sum_fst ((x, a) :: l) = x + sum_fst l.