proposition: Lemma big_seq_cond (I : eqType) r (P : pred I) F : \big[op/idx]_(i <- r | P i) F i = \big[op/idx]_(i <- r | (i \in r) && P i) F i.
proof: Proof. by rewrite -!(big_filter r); congr bigop; apply: eq_in_filter => i ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)