simp only [← List.mem_toFinset] at a1
case h
z : String
t : Var
X : String
vs : List Var
v : Var
a1 : v ∈ vs
⊢ v.freeVarSet \ {free_ z} ⊆ vs.toFinset.biUnion Var.freeVarSet \ {free_ z}
case h
z : String
t : Var
X : String
vs : List Var
v : Var
a1 : v ∈ vs.toFinset
⊢ v.freeVarSet \ {free_ z} ⊆ vs.toFinset.biUnion Var.freeVarSet \ {free_ z}