diff --git a/todo.md b/todo.md
index 0d59b74..9fb48b6 100644
--- a/todo.md
+++ b/todo.md
@@ -11,3 +11,4 @@
   - batch
 - optimize `indexes()` double-diffing
 - define behavior of deriving a source within a derived source
+- review destruction of node under a root that has a child in another root