diff --git a/masque/library.py b/masque/library.py
index 1bb5e90..f25c032 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -1025,7 +1025,7 @@ class Tree(MutableLibrary):
     def __delitem__(self, key: str) -> None:
         del self.library[key]
 
-    def __iadd__(self, other: 'Tree') -> None:
+    def __lshift__(self, other: 'Tree') -> None:
         self.add_tree(other)