Update todo

todo.md

@@ -8,6 +8,27 @@
   - Portal
   - batch
 - optimize `indexes()` double-diffing
-- define behavior of deriving a source within a derived source
-- define behavior of node destruction under a root that has a child in another root
 - improve crash course, some sections feel like information dumps
+- reactive scopes
+  - define behavior of creating a reactive scope within a non-root reactive scope
+    - i.e. an effect within an effect
+    - error on attempt for now
+  - define behavior of destruction of a node with a child, where the node and
+    its child are in separate, non-nested root reactive scopes.
+    - should destruction of the parent also destroy the child?
+    - or should destruction of the parent silently disconnect the child, without
+      invoking the child's cleanups?
+
+    ```mermaid
+    graph LR
+
+    subgraph root1
+        parent
+    end
+
+    subgraph root2
+        child
+    end
+
+    parent --> child
+    ```