Index: branches/pyyaml3000/lib/yaml/constructor.py
===================================================================
--- branches/pyyaml3000/lib/yaml/constructor.py	(revision 56)
+++ branches/pyyaml3000/lib/yaml/constructor.py	(revision 57)
@@ -1,2 +1,5 @@
+
+__all__ = ['BaseConstructor', 'Constructor', 'ConstructorError',
+    'YAMLObject', 'YAMLObjectMetaclass']
 
 from error import *
@@ -325,4 +328,9 @@
         return self.construct_mapping(node)
 
+    def construct_undefined(self, node):
+        raise ConstructorError(None, None,
+                "could not determine a constructor for the tag %r" % node.tag.encode('utf-8'),
+                node.start_marker)
+
 Constructor.add_constructor(
         u'tag:yaml.org,2002:null',
@@ -368,4 +376,7 @@
         u'tag:yaml.org,2002:map',
         Constructor.construct_yaml_map)
+
+Constructor.add_constructor(None,
+        Constructor.construct_undefined)
 
 class YAMLObjectMetaclass(type):