simp only [replaceFreeAux]
case def_
σ σ' : VarName → VarName
a✝¹ : DefName
a✝ : List VarName
binders : Finset VarName
h1 : ∀ v ∉ binders, σ v = σ' v
⊢ replaceFreeAux σ binders (def_ a✝¹ a✝) = replaceFreeAux σ' binders (def_ a✝¹ a✝)
case def_
σ σ' : VarName → VarName
a✝¹ : DefName
a✝ : List VarName
binders : Finset VarName
h1 : ∀ v ∉ binders, σ v = σ' v
⊢ def_ a✝¹ (List.map (fun x => if x ∉ binders then σ x else x) a✝) =
    def_ a✝¹ (List.map (fun x => if x ∉ binders then σ' x else x) a✝)