simp [bind, interpWriter]
case Ret
E F : Type → Type
T R : Type
h : E ~> WriterT (Fitree F)
k : T → Fitree E R
r✝ : T
⊢ interpWriter h (bind (Ret r✝) k) =
    bind (interpWriter h (Ret r✝)) fun x =>
      match x with
      | (x, s₁) =>
        bind (interpWriter h (k x)) fun x =>
          match x with
          | (y, s₂) => ret (y, s₁ ++ s₂)
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)