Arguments cardD1x [I A].

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