diff --git a/naive-nlu/tree_nlu/knowledge_evaluation.py b/naive-nlu/tree_nlu/knowledge_evaluation.py
index 4a49faa..5cc4f65 100644
--- a/naive-nlu/tree_nlu/knowledge_evaluation.py
+++ b/naive-nlu/tree_nlu/knowledge_evaluation.py
@@ -113,6 +113,21 @@ def modifiable_element_for_existance_in_set(container, set_name, element):
         setter=setter,
     )
 
+def modifiable_element_for_existance_in_group(container, element, backlink, set_name='groups'):
+    def getter():
+        nonlocal container, element, backlink, set_name
+        return (set_name in container) and (element in container[set_name])
+
+    def setter():
+        nonlocal container, set_name, element
+        backlink['groups'].add(set_name)
+        return container[set_name].add(element)
+
+    return ModifiableProperty(
+        getter=getter,
+        setter=setter,
+    )
+
 def pertenence_to_group(knowledge_base, elements, subj, group):
     subj = resolve(knowledge_base, elements, subj)
     group = resolve(knowledge_base, elements, group)