proposition: Lemma neqr0_sign x : x != 0 -> (-1) ^+ (x < 0)%R = sgr x.
proof: Proof. by rewrite sgr_def => ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)