proposition: Lemma ffactnn n : n ^_ n = n`!.
proof: Proof. by elim: n => [|n IHn] //; rewrite ffactnS IHn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div fintype tuple finfun bigop prime finset)