diff --git a/tiramisu/config.py b/tiramisu/config.py
index 58e9067..ff6595d 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -57,11 +57,13 @@ class Config(object):
         if parent is None:
             self._cfgimpl_settings = Setting()
             self._cfgimpl_values = Values(self._cfgimpl_context)
+            self._cfgimpl_all_paths = {}
         else:
             if context is None:
                 raise ConfigError("cannot find a value for this config")
             self._cfgimpl_settings = None
             self._cfgimpl_values = None
+            self._cfgimpl_all_paths = None
         "warnings are a great idea, let's make up a better use of it"
         self._cfgimpl_warnings = []
         self._cfgimpl_toplevel = self._cfgimpl_get_toplevel()
@@ -70,6 +72,13 @@ class Config(object):
         self._cfgimpl_slots = [key for key, value in methods
                               if not key.startswith("_")]
         self._cfgimpl_build()
+        if parent is None:
+            self._cfgimpl_build_all_paths()
+
+    def _cfgimpl_build_all_paths(self):
+        if self._cfgimpl_all_paths == None:
+            raise ConfigError('cache paths must not be None')
+        self._cfgimpl_descr.build_cache(self._cfgimpl_all_paths)
 
     def cfgimpl_get_settings(self):
         return self._cfgimpl_context._cfgimpl_settings
@@ -326,7 +335,16 @@ class Config(object):
             subpath.insert(0, obj._cfgimpl_descr._name)
             obj = obj._cfgimpl_parent
         return ".".join(subpath)
+    # ______________________________________________________________________
+#    def cfgimpl_previous_value(self, path):
+#        "stores the previous value"
+#        home, name = self._cfgimpl_get_home_by_path(path)
+#        # FIXME  fucking name
+#        return home._cfgimpl_context._cfgimpl_values.previous_values[name]
 
+#    def get_previous_value(self, name):
+#        "for the time being, only the previous Option's value is accessible"
+#        return self._cfgimpl_context._cfgimpl_values.previous_values[name]
     # ______________________________________________________________________
     def add_warning(self, warning):
         "Config implements its own warning pile. Could be useful"
@@ -413,17 +431,19 @@ class Config(object):
         """
         paths = []
         for path in self._cfgimpl_descr.getpaths(include_groups=include_groups):
-            try:
-                value = getattr(self, path)
-
-            except MandatoryError:
-                if mandatory or allpaths:
-                    paths.append(path)
-            except PropertiesOptionError:
-                if allpaths:
-                    paths.append(path)  # option which have properties added
-            else:
+            if allpaths:
                 paths.append(path)
+            else:
+                try:
+                    value = getattr(self, path)
+
+                except MandatoryError:
+                    if mandatory:
+                        paths.append(path)
+                except PropertiesOptionError:
+                    pass
+                else:
+                    paths.append(path)
         return paths
 
     def _find(self, bytype, byname, byvalue, byattrs, first):
diff --git a/tiramisu/option.py b/tiramisu/option.py
index d43d7f8..02dd613 100644
--- a/tiramisu/option.py
+++ b/tiramisu/option.py
@@ -444,6 +444,21 @@ class OptionDescription(BaseType):
             else:
                 paths.append('.'.join(currpath + [attr]))
         return paths
+
+    def build_cache(self, paths, currpath=None):
+        if currpath is None:
+            currpath = []
+        for option in self._children:
+            attr = option._name
+            if attr.startswith('_cfgimpl'):
+                continue
+            if isinstance(option, OptionDescription):
+                currpath.append(attr)
+                option.build_cache(paths, currpath=currpath)
+                currpath.pop()
+            else:
+                paths[option] = '.'.join(currpath + [attr])
+
     # ____________________________________________________________
     def set_group_type(self, group_type):
         """sets a given group object to an OptionDescription
diff --git a/tiramisu/value.py b/tiramisu/value.py
index cd256fc..db3b8f8 100644
--- a/tiramisu/value.py
+++ b/tiramisu/value.py
@@ -55,9 +55,18 @@ class Values(object):
         if opt not in self.values:
             if opt.is_multi():
                 multitype = self._get_multitype(opt)
-                return Multi(opt.getdefault(), self.context, opt, multitype)
+                value = Multi(opt.getdefault(), self.context, opt, multitype)
             else:
-                return opt.getdefault()
+                value = opt.getdefault()
+
+            if opt in self.slaves:
+                masterpath = self.context._cfgimpl_all_paths[self.slaves[opt]]
+                mastervalue = getattr(self.context, masterpath)
+                masterlen = len(mastervalue)
+                if len(value) < masterlen:
+                    for num in range(0, masterlen - len(value)):
+                        value.append(None, force=True)
+            return value
         return self.values[opt]
 
     def reset(self, opt):