Important behavior change : to add default_multi value, now use Multi.append(), not Multi.append(None)

tiramisu/setting.py

@@ -242,6 +242,12 @@ multitypes = MultiTypeModule()
 populate_multitypes()
 
 
+# ____________________________________________________________
+class Undefined():
+    pass
+
+
+undefined = Undefined()
 # ____________________________________________________________
 class Property(object):
     "a property is responsible of the option's value access rules"