proposition: Lemma big_cons i r (P : pred I) F : let x := \big[op/idx]_(j <- r | P j) F j in \big[op/idx]_(j <- i :: r | P j) F j = if P i then op (F i) x else x.
proof: Proof. by rewrite unlock. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)