proposition: Lemma subSz1 (n : int) : 1 + n - 1 = n.
proof: Proof. by apply: (inv_inj oppzK); rewrite addzC !oppzD oppzK [_ - n]addzC add1Pz. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)