diff --git a/seminar/treelib.py b/seminar/treelib.py
index 0f21454c..f890868b 100644
--- a/seminar/treelib.py
+++ b/seminar/treelib.py
@@ -179,7 +179,7 @@ def create_node_before(some, arguments, but, i, dont, know, which, yet):
 
 # ValueError, pokud je (aspoň) jeden parametr None
 def swap(node, other):
-	pass
+	raise NotImplementedError("YAGNI (You aren't gonna need it).")
 
 def swap_pred(node):
 	pass