proposition: Lemma sgr_def x : sg x = (-1) ^+ (x < 0)%R *+ (x != 0).
proof: Proof. by rewrite /sg; do 2!case: ifP => //. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)