diff --git a/Concurrent Systems/notes/7- MUTEX-free concurrency.md b/Concurrent Systems/notes/7- MUTEX-free concurrency.md
index 6ec93e1..07f7a62 100644
--- a/Concurrent Systems/notes/7- MUTEX-free concurrency.md	
+++ b/Concurrent Systems/notes/7- MUTEX-free concurrency.md	
@@ -138,4 +138,13 @@ Idea: every operation is started by the invoking process and finalized by the ne
 
 This is needed for the so called ABA problem with compare&set:
 - A typical use of compare&set is
-  ```stm```
\ No newline at end of file
+```
+	stmp <- X
+	...
+	if X.compare&set(tmp, v) then ...
+```
+- this is to ensure that the value of X has not changed in the computation
+- the problem is that X can be changed twice before compare&set
+- solution: X is a pair ⟨val , seq_numb⟩, with the constraint that each modification of X increases its sequence_number
+	- with the compare&set you mainly test that the sequence_number has not changed
+