Index: /pyyaml/trunk/lib/yaml/__init__.py
===================================================================
--- /pyyaml/trunk/lib/yaml/__init__.py	(revision 223)
+++ /pyyaml/trunk/lib/yaml/__init__.py	(revision 252)
@@ -266,4 +266,5 @@
 
     __metaclass__ = YAMLObjectMetaclass
+    __slots__ = ()  # no direct instantiation, so allow immutable subclasses
 
     yaml_loader = Loader