proposition: Lemma nbsurj_nn n: 'Sj(n, n) = n`!.
proof: Proof. by rewrite /nbsurj stir_nn mul1n. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)