proposition: Lemma derangeS n: derange n.+2 = (n.+1) * (derange n + derange n.+1).
proof: Proof. by []. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)