simp only [Rec.fastReplaceFree]
case def_
v u : VarName
a✝¹ : DefName
a✝ : List VarName
binders : Finset VarName
h1 : v ∈ a✝ → u ∉ binders
⊢ IsSub (def_ a✝¹ a✝) v u (Rec.fastReplaceFree v u (def_ a✝¹ a✝))
case def_
v u : VarName
a✝¹ : DefName
a✝ : List VarName
binders : Finset VarName
h1 : v ∈ a✝ → u ∉ binders
⊢ IsSub (def_ a✝¹ a✝) v u (def_ a✝¹ (List.map (fun x => if v = x then u else x) a✝))