diff --git a/ir.py b/ir.py
index 0e71a97..182092e 100644
--- a/ir.py
+++ b/ir.py
@@ -3,7 +3,6 @@
 # The COPYRIGHT file at the top level of this repository contains the
 # full copyright notices and license terms.
 
-from trytond.transaction import Transaction
 from trytond.pool import PoolMeta
 
 
@@ -14,7 +13,9 @@ class Rule(metaclass=PoolMeta):
     def _context_modelnames(cls):
         """ list of models to add 'user_id' to context
         """
-        return super(Rule, cls)._context_modelnames() + [
-            'cashbook.bookcategory']
+        result = super(Rule, cls)._context_modelnames()
+        return result | {
+            'cashbook.bookcategory',
+        }
 
 # end Rule