diff --git a/Concurrent Systems/notes/9 - Consensus.md b/Concurrent Systems/notes/9 - Consensus.md
index fe4c083..60120dc 100644
--- a/Concurrent Systems/notes/9 - Consensus.md	
+++ b/Concurrent Systems/notes/9 - Consensus.md	
@@ -20,4 +20,20 @@ This object is a **consensus object**: a *one-shot object* (every process can ac
 - **Validity:** the returned value (or *decided value*) is one of the arguments of the propose (*proposed value*) in one invocation done by a process (*participant*)
 - **Integrity:** every process decides at most once
 - **Agreement:** the decided value is the same for all processes
-- **Wait-freedom:** every invocation of propose by a correct process terminates
\ No newline at end of file
+- **Wait-freedom:** every invocation of propose by a correct process terminates
+
+Conceptually, we can implement a consensus object by a register X, initialized at ⊥, for which the propose operation is atomically defined as:
+```
+propose(v) :=
+	if X = ⊥ then
+		X <- v
+		return X
+```
+
+Universality of consensus holds as folows:
+- given an object O of type Z
+- each participant runs a local copy of O, all initialized at the same value
+- create a total order on the operations O, by using consensus objects
+- force all processes to follow this order to locally simulate O
+	- all local copies are consistent
+