proposition: Lemma big_filter r (P : pred I) F : \big[op/idx]_(i <- filter P r) F i = \big[op/idx]_(i <- r | P i) F i.
proof: Proof. by rewrite unlock; elim: r => //= i r <-; case (P i). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)