proposition: Lemma rangeFreeVars_ALFilter_subset : forall (sub : SSubstitution vc) l, subset (rangeFreeVars (SubFilter sub l)) (rangeFreeVars sub).
proof: Proof. introv i. allunfold rangeFreeVars. allrw lin_flat_map; exrepnd. exists x0; sp. allapply in_ALRange_ALFilter_implies; sp. Qed.
imports: Term AssociationList