proposition: Lemma big_has_cond : \big[orb/false]_(i <- r | P i) B i = has (predI P B) r.
proof: Proof. by rewrite big_mkcond unlock. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)