proposition: Lemma big_catr r1 r2 (P : pred I) F : ~~ has P r1 -> \big[op/idx]_(i <- r1 ++ r2 | P i) F i = \big[op/idx]_(i <- r2 | P i) F i.
proof: Proof. rewrite -big_filter -(big_filter r2) filter_cat. by rewrite has_count -size_filter; case: filter. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)