congr! 1
σ σ' : VarName → VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : ∀ v ∉ binders, σ v = σ' v
⊢ def_ X (List.map (fun x => if x ∉ binders then σ x else x) xs) =
    def_ X (List.map (fun x => if x ∉ binders then σ' x else x) xs)
case h.e'_2
σ σ' : VarName → VarName
X : DefName
xs : List VarName
binders : Finset VarName
h1 : ∀ v ∉ binders, σ v = σ' v
⊢ List.map (fun x => if x ∉ binders then σ x else x) xs = List.map (fun x => if x ∉ binders then σ' x else x) xs