simp [interpUB, Fitree.interpExcept] at *
case Vis
T R : Type
k : T → Fitree UBE R
T✝ : Type
e✝ : UBE T✝
k✝ : T✝ → Fitree UBE T
ih :
  ∀ (a : T✝),
    interpUB (Fitree.bind (k✝ a) k) =
      Fitree.bind (interpUB (k✝ a)) fun x =>
        match x with
        | Except.error ε => Fitree.ret (Except.error ε)
        | Except.ok x => interpUB (k x)
⊢ interpUB (Fitree.bind (Fitree.Vis e✝ k✝) k) =
    Fitree.bind (interpUB (Fitree.Vis e✝ k✝)) fun x =>
      match x with
      | Except.error ε => Fitree.ret (Except.error ε)
      | Except.ok x => interpUB (k x)
case Vis
T R : Type
k : T → Fitree UBE R
T✝ : Type
e✝ : UBE T✝
k✝ : T✝ → Fitree UBE T
ih :
  ∀ (a : T✝),
    Fitree.interp UBE.handle (Fitree.bind (k✝ a) k) =
      Fitree.bind (Fitree.interp UBE.handle (k✝ a)) fun x =>
        match x with
        | Except.error ε => Fitree.ret (Except.error ε)
        | Except.ok x => Fitree.interp UBE.handle (k x)
⊢ Fitree.interp UBE.handle (Fitree.bind (Fitree.Vis e✝ k✝) k) =
    Fitree.bind (Fitree.interp UBE.handle (Fitree.Vis e✝ k✝)) fun x =>
      match x with
      | Except.error ε => Fitree.ret (Except.error ε)
      | Except.ok x => Fitree.interp UBE.handle (k x)