proposition: Lemma catfAC f g h : f + g + h = f + h + g.[\ domf 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)