∀ {m : Type u_1 → Type u_2} {α β : Type u_1} [inst : Monad m] [inst_1 : MonadAttach m] {x : m α}
  {f : Subtype (MonadAttach.CanReturn x) → m β}, MonadAttach.attach x >>= f = MonadAttach.pbind x fun a ha => f ⟨a, ha⟩