proposition: Lemma reindex_inj (I : finType) (h : I -> I) (P : pred I) F : injective h -> \big[op/x]_(i | P i) F i = \big[op/x]_(j | P (h j)) F (h j).
proof: Proof. by move=> injh; apply: reindex (onW_bij _ (injF_bij injh)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)