proposition: Lemma big_add1 m n (P : pred nat) F : \big[op/idx]_(m.+1 <= i < n | P i) F i = \big[op/idx]_(m <= i < n.-1 | P (i.+1)) F (i.+1).
proof: Proof. by rewrite -addn1 big_addn subn1; apply: eq_big => ? *; rewrite addn1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)