--- a/schema.py	Mon Jul 15 10:59:34 2013 +0200
+++ b/schema.py	Mon Jul 22 14:26:33 2013 +0200
@@ -711,6 +711,9 @@
             return self.expression == other.expression
         return False
 
+    def __hash__(self):
+        return hash(self.expression)
+
     def __deepcopy__(self, memo):
         return self.__class__(self.expression, self.mainvars)
     def __getstate__(self):