.

wcfs/internal/xbtree/δbtail.go

@@ -773,6 +773,11 @@ func (δBtail *ΔBtail) GetAt(ctx context.Context, root *Tree, key Key, at zodb.
 
 	// XXX dirty -> rebuild
 
+	rootAt := root.PJar().At()
+	if rootAt != δBtail.Head() {
+		panicf("δBtail: root.at (@%s)  !=  head (@%s)", rootAt, δBtail.Head())
+	}
+
 	δTtail := δBtail.vδTbyRoot[root.POid()]
 	if δTtail == nil {
 		panicf("δBtail: root<%s> not tracked", root.POid())