diff --git a/lib/outlook/internal_tree/internal_tree.ex b/lib/outlook/internal_tree/internal_tree.ex
index b3e3166..08e1d69 100644
--- a/lib/outlook/internal_tree/internal_tree.ex
+++ b/lib/outlook/internal_tree/internal_tree.ex
@@ -58,4 +58,19 @@ defmodule Outlook.InternalTree.InternalTree do
     |> Enum.into(node_atts)
     %{node | eph: Map.put(node.eph, :attributes, attributes)}
   end
+
+
+  def collect_tunit_ids([%TranslationUnit{} = node | rest]) do
+    [node.nid | collect_tunit_ids(rest)]
+  end
+
+  def collect_tunit_ids([%{type: :element} = node | rest]) do
+    collect_tunit_ids(node.content) ++ collect_tunit_ids(rest)
+  end
+
+  def collect_tunit_ids([node | rest]) do
+    collect_tunit_ids(rest)
+  end
+
+  def collect_tunit_ids([]), do: []
 end