proposition: Lemma big_const_idem I (r : seq I) P : \big[op/x]_(i <- r | P i) x = x.
proof: Proof. by elim/big_ind : _ => // _ _ -> ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div fintype tuple finfun)