     case res do
       txt when is_binary(txt) -> txt
       num when is_number(num) -> to_string(num)
+      :unknown_var -> "{{#{var}}}"
       other -> inspect other
     end
   end
 
   def re_find_and_replace_leaf(var, nil) do
-    "[UNKNOWN #{var}]"
+    :unknown_var
+  end
+  def re_find_and_replace_leaf(var, :unknown_var) do
+    :unknown_var
   end
   def re_find_and_replace_leaf(_var, s) when is_binary(s) do
     s

    {:ok, res}
  end

  def re_find_and_replace("", context) do # nice for debugging: {{}}
    inspect context
  end

        _ -> nil
      end)
    case maybe_var do
      nil -> :unknown_var
      {_k, v} -> v
    end
  end
