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