have h₁: forall x, "" ++ x = x := by
  simp [HAppend.hAppend, Append.append, String.append]
  simp [List.nil_append]
case Ret
E F : Type → Type
T R : Type
h : E ~> WriterT (Fitree F)
k : T → Fitree E R
r✝ : T
⊢ interp h (k r✝) = bind (interp h (k r✝)) fun x => ret (x.fst, "" ++ x.snd)
case Ret
E F : Type → Type
T R : Type
h : E ~> WriterT (Fitree F)
k : T → Fitree E R
r✝ : T
h₁ : ∀ (x : String), "" ++ x = x
⊢ interp h (k r✝) = bind (interp h (k r✝)) fun x => ret (x.fst, "" ++ x.snd)