proposition: Lemma ffact_gt0 n m : (0 < n ^_ m) = (m <= n).
proof: Proof. by elim: n m => [|n IHn] [|m] //=; rewrite ffactSS muln_gt0 IHn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div fintype tuple finfun bigop prime finset)