apply Finset.union_subset_union_left
zs : List String
X : String
vs : List Var
j : ℕ
v : Var
a1 : v ∈ vs
⊢ v.freeVarSet ∪ (List.map free_ zs).toFinset ⊆ vs.toFinset.biUnion Var.freeVarSet ∪ (List.map free_ zs).toFinset
case h
zs : List String
X : String
vs : List Var
j : ℕ
v : Var
a1 : v ∈ vs
⊢ v.freeVarSet ⊆ vs.toFinset.biUnion Var.freeVarSet