cases v
z : String
t v : Var
⊢ v.freeVarSet \ {free_ z} ⊆ ((free_ z).subst t v).freeVarSet
case free_
z : String
t : Var
a✝ : String
⊢ (free_ a✝).freeVarSet \ {free_ z} ⊆ ((free_ z).subst t (free_ a✝)).freeVarSet

case bound_
z : String
t : Var
a✝ : ℕ
⊢ (bound_ a✝).freeVarSet \ {free_ z} ⊆ ((free_ z).subst t (bound_ a✝)).freeVarSet