proposition: Lemma pair_big_idem (I J : finType) (P : pred I) (Q : pred J) F : \big[op/x]_(i | P i) \big[op/x]_(j | Q j) F i j = \big[op/x]_(p | P p.1 && Q p.2) F p.1 p.2.
proof: Proof. exact: pair_big_dep_idem. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)