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