Use lshift for tree combination

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)