proposition: Lemma sgz_id (x : R) : sgz (sgz x) = sgz x.
proof: Proof. by rewrite !(fun_if (@sgz _)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)