--- a/schema.py	Tue Sep 08 17:15:03 2015 +0200
+++ b/schema.py	Tue Sep 08 18:04:57 2015 +0200
@@ -253,6 +253,9 @@
             return self.expression == other.expression
         return False
 
+    def __ne__(self, other):
+        return not (self == other)
+
     def __hash__(self):
         return hash(self.expression)