rfl
case Ret
T : Type
E : Type → Type
R : Type
k : T → Fitree (UBE +' E) R
r✝ : T
⊢ interpUB' (Fitree.bind (Fitree.Ret r✝) k) =
    Fitree.bind (interpUB' (Fitree.Ret r✝)) fun x =>
      match x with
      | Except.error ε => Fitree.ret (Except.error ε)
      | Except.ok x => interpUB' (k x)
no goals