Add explanation of why this work correctly.

theano/gof/type.py

@@ -403,9 +403,11 @@ class SingletonType(Type):
     __instance = None
 
     def __new__(cls):
-        # I don't understand why the subclass of subclass of
-        # SingletonType would share the __instance. So I add the check
-        # for the type.
+        # If sub-subclass of SingletonType don't redeclare __instance
+        # when we look for it, we will find it in the subclass.  We
+        # don't want that, so we check the class.  When we add one, we
+        # add one only to the current class, so all is working
+        # correctly.
         if cls.__instance is None or not isinstance(cls.__instance, cls):
             cls.__instance = Type.__new__(cls)
         return cls.__instance