diff --git a/ir.py b/ir.py
index 1573ebf..0a74a0a 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
 
 
@@ -12,10 +11,16 @@ class Rule(metaclass=PoolMeta):
 
     @classmethod
     def _context_modelnames(cls):
-        """ list of models to add 'user_id' to context
         """
-        return super(Rule, cls)._context_modelnames() + [
-            'ir.action.act_window', 'cashbook_report.evaluation']
+
+        Returns:
+            set: model-names
+        """
+        result = super(Rule, cls)._context_modelnames()
+        return result | {
+            'ir.action.act_window',
+            'cashbook_report.evaluation',
+            }
 
 # end Rule