diff --git a/todo.yaml b/todo.yaml
index 776787e..d661b8f 100755
--- a/todo.yaml
+++ b/todo.yaml
@@ -1,4 +1,5 @@
 todo:
+- https://en.wikipedia.org/wiki/SuperMemo#Description_of_SM-2_algorithm
 - exemplary answers; declaring
 - exemplary answers; sharing
 - tags; declare mastery