proposition: Lemma exprnN x (n : nat) : x ^- n = x ^ -n%:Z.
proof: Proof. by case: n=> //; rewrite oppr0 expr0 invr1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)