proposition: Lemma big_mask I r m (P : pred I) (F : I -> R) (r_ := tnth (in_tuple r)) : \big[op/idx]_(i <- mask m r | P i) F i = \big[op/idx]_(i < size r | nth false m i && P (r_ i)) F (r_ i).
proof: Proof. exact: (big_mask_tuple _ (in_tuple r)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)