proposition: Lemma sgr_eq0 x : (sg x == 0) = (x == 0).
proof: Proof. by rewrite -sqrf_eq0 sqr_sg pnatr_eq0; case: (x == 0). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)