proposition: Lemma catfCA f g h : f + (g + h) = g + (f.[\ domf g] + h).
proof: Proof. by rewrite !catfA [X in X + _]catfC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)