proposition: Lemma pair_big_dep (I J : finType) (P : pred I) (Q : I -> pred J) F : \big[*%M/1]_(i | P i) \big[*%M/1]_(j | Q i j) F i j = \big[*%M/1]_(p | P p.1 && Q p.1 p.2) F p.1 p.2.
proof: Proof. exact/pair_big_dep_idem/mul1m. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)