proposition: Fact poly_inv_out : {in [predC poly_unit], poly_inv =1 id}.
proof: Proof. by rewrite /poly_inv => p /negbTE/= ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype bigop finset tuple) mathcomp(div ssralg countalg binomial)