Arguments partition_big [I J P] p Q [F].

Lemma reindex_onto (I J : finType) (h : J -> I) h' (P : pred I) F :
(forall i, P i -> h (h' i) = i) ->
\big[*%M/1]_(i | P i) F i =
\big[*%M/1]_(j | P (h j) && (h' (h j) == j)) F (h j).