diff --git a/xmind/core/const.py b/xmind/core/const.py
index a9b27d1..e637c01 100644
--- a/xmind/core/const.py
+++ b/xmind/core/const.py
@@ -67,6 +67,7 @@
 
 TOPIC_ROOT = "root"
 TOPIC_ATTACHED = "attached"
+TOPIC_DETACHED = "detached"
 
 FILE_PROTOCOL = "file://"
 TOPIC_PROTOCOL = "xmind:#"