proposition: Lemma reindex_onto (I J : finType) (h : J -> I) h' (P : pred I) F : (forall i, P i -> h (h' i) = i) -> \big[op/x]_(i | P i) F i = \big[op/x]_(j | P (h j) && (h' (h j) == j)) F (h j).
proof: Proof. by move=> h'K; rewrite (reindex_omap h (some \o h'))//= => i Pi; rewrite h'K. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)