Arguments reindex_onto [I J] h h' [P F].

(* Specifications: *)
Lemma reindex (I J : finType) (h : J -> I) (P : pred I) F :
{on [pred i | P i], bijective h} ->
\big[*%M/1]_(i | P i) F i = \big[*%M/1]_(j | P (h j)) F (h j).