    = mapDict toBackend
    . mapDict (Sub (Impl.inferBackendInstance @t @ds))
    $ inferDeriveContext @t @ds @b undefined

inferNum
  :: forall (t :: Type) (ds :: [Nat]) (b :: Type) (i :: Type)
   . (Num t, Impl.KnownBackend t ds b)
  => Dict (Num (Backend i t ds b))
inferNum
    = mapDict toBackend