   @doc """
   Get the parent of the focus node, if any.
   """
-  @spec up(Z.t()) :: Z.t()
+  @spec up(Z.t()) :: Z.t() | nil
   def up(z) do
   end
 

  @doc """
  Replace the left child tree of the focus node.
  """
  @spec set_left(Z.t(), BT.t()) :: Z.t()
  def set_left(z, l) do
  end

  @doc """
  Replace the right child tree of the focus node.
  """
  @spec set_right(Z.t(), BT.t()) :: Z.t()
  def set_right(z, r) do
  end
end

end

defmodule Zipper do
  alias BinTree, as: BT
  alias Zipper, as: Z

  @doc """
  Get a zipper focused on the root node.
  """
