proposition: Lemma big_cat_nested r1 r2 (P : pred I) F : let x := \big[op/idx]_(i <- r2 | P i) F i in \big[op/idx]_(i <- r1 ++ r2 | P i) F i = \big[op/x]_(i <- r1 | P i) F i.
proof: Proof. by rewrite unlock /reducebig foldr_cat. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)