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