diff --git a/tiramisu/setting.py b/tiramisu/setting.py
index 46741b5..c2ca92f 100644
--- a/tiramisu/setting.py
+++ b/tiramisu/setting.py
@@ -30,11 +30,11 @@ from tiramisu.i18n import _
 
 default_encoding = 'utf-8'
 expires_time = 5
-ro_remove = ('permissive', 'hidden')
-ro_append = ('frozen', 'disabled', 'validator', 'everything_frozen',
-             'mandatory')
-rw_remove = ('permissive', 'everything_frozen', 'mandatory')
-rw_append = ('frozen', 'disabled', 'validator', 'hidden')
+ro_remove = set('permissive', 'hidden')
+ro_append = set('frozen', 'disabled', 'validator', 'everything_frozen',
+                'mandatory')
+rw_remove = set('permissive', 'everything_frozen', 'mandatory')
+rw_append = set('frozen', 'disabled', 'validator', 'hidden')
 default_properties = ('expire', 'validator')