 end
 
 defmodule Zipper do
+  alias BinTree, as: BT
+  alias Zipper, as: Z
+
   @doc """
   Get a zipper focused on the root node.
   """

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


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