proposition: Lemma oppr_itvoo (a b x : R) : (- x \in `]a, b[) = (x \in `](- b), (- a)[).
proof: Proof. exact: oppr_itv. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(div fintype bigop order ssralg finset fingroup) mathcomp(ssrnum)