proposition: Lemma csum_An2 I J (f: fterm2): csumb I (fun i => csumb J (fun j => f i j)) = csumb J (fun j => csumb I (fun i => f i j)).
proof: Proof. exact: (proj1 (csum_prod_assoc_aux I J f)). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)