proposition: Lemma big_addn m n a (P : pred nat) F : \big[op/idx]_(m + a <= i < n | P i) F i = \big[op/idx]_(m <= i < n - a | P (i + a)) F (i + a).
proof: Proof. rewrite /index_iota -subnDA addnC iotaDl big_map. by apply: eq_big => ? *; rewrite addnC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)