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