proposition: Lemma itv_xx x cl cr y : y \in Interval (BSide cl x) (BSide cr x) = cl && ~~ cr && (y == x).
proof: Proof. by case: cl cr => [] []; rewrite [LHS]lteif_anti // eq_sym. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(div fintype bigop order ssralg finset fingroup) mathcomp(ssrnum)