diff --git a/lifetimes.md b/lifetimes.md
index 48bfcb0..cd6f5ad 100644
--- a/lifetimes.md
+++ b/lifetimes.md
@@ -9,6 +9,8 @@ language-design problem.
 
 # The Tagged Union Problem
 
+TODO: rewrite this to use Box instead?
+
 The core of the lifetime and mutability system derives from a simple problem:
 internal pointers to tagged unions. For instance, consider the following code: