proposition: Lemma sqr_sg x : sg x ^+ 2 = (x != 0)%:R.
proof: Proof. by rewrite sgr_def exprMn_n sqrr_sign -mulnn mulnb andbb. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)