diff --git a/tiramisu/config.py b/tiramisu/config.py
index f3701cb..7ad800a 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -50,6 +50,7 @@ class Config(object):
         # sub option descriptions
         self._cfgimpl_subconfigs = {}
         self._cfgimpl_parent = parent
+        self._cfgimpl_permissive = []
         if context is None:
             self._cfgimpl_context = self
         else:
@@ -168,14 +169,12 @@ class Config(object):
         if not isinstance(opt_or_descr, Option) and \
                 not isinstance(opt_or_descr, OptionDescription):
             raise TypeError('Unexpected object: {0}'.format(repr(opt_or_descr)))
-        properties = copy(opt_or_descr.properties)
-        for proper in copy(properties):
-            if not self._cfgimpl_context._cfgimpl_settings.has_property(proper):
-                properties.remove(proper)
+        properties = set(copy(opt_or_descr.properties))
+        properties = properties & set(self._cfgimpl_context._cfgimpl_settings.get_properties())
         if permissive:
-            for perm in self._cfgimpl_context._cfgimpl_settings.permissive:
-                if perm in properties:
-                    properties.remove(perm)
+            properties = properties - set(self._cfgimpl_context._cfgimpl_settings.get_permissive())
+            properties = properties - set(self._cfgimpl_permissive)
+        properties = list(properties)
         if properties != []:
             raise PropertiesOptionError("trying to access"
                     " to an option named: {0} with properties"
@@ -415,6 +414,11 @@ class Config(object):
                 except:
                     pass
     # ______________________________________________________________________
+    def cfgimpl_set_permissive(self, permissive):
+        if not isinstance(permissive, list):
+            raise TypeError('permissive must be a list')
+        self._cfgimpl_permissive = permissive
+    # ______________________________________________________________________
     def __str__(self):
         "Config's string representation"
         lines = []
diff --git a/tiramisu/setting.py b/tiramisu/setting.py
index 0e989c6..6bdcb13 100644
--- a/tiramisu/setting.py
+++ b/tiramisu/setting.py
@@ -137,6 +137,10 @@ class Setting():
         "has properties means the Config's properties attribute is not empty"
         return bool(len(self.properties))
 
+
+    def get_properties(self):
+        return self.properties
+
     def has_property(self, propname):
         """has property propname in the Config's properties attribute
         :param property: string wich is the name of the property"""
@@ -152,6 +156,9 @@ class Setting():
         if self.has_property(propname):
             self.properties.remove(propname)
     #____________________________________________________________
+    def get_permissive(self):
+        return self.permissive
+
     def set_permissive(self, permissive):
         if not isinstance(permissive, list):
             raise TypeError('permissive must be a list')