proposition: Lemma ltr0_sg x : x < 0 -> sg x = -1.
proof: Proof. by move=> x_lt0; rewrite /sg x_lt0 lt_eqF. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)