proposition: Lemma big_mask_tuple I n m (t : n.-tuple I) (P : pred I) F : \big[op/idx]_(i <- mask m t | P i) F i = \big[op/idx]_(i < n | nth false m i && P (tnth t i)) F (tnth t i).
proof: Proof. rewrite [t in LHS]tuple_map_ord/= -map_mask big_map. by rewrite mask_enum_ord big_filter_cond/= enumT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)