diff --git a/4a1s/MFES/PL - Aula 4.md b/4a1s/MFES/PL - Aula 4.md
index a2a49ce..92be624 100644
--- a/4a1s/MFES/PL - Aula 4.md	
+++ b/4a1s/MFES/PL - Aula 4.md	
@@ -26,20 +26,30 @@ sig Day {}
 pred inv1 {
 	// Every image is posted by one user.
 	all y: Photo | some x: User | x->y in posts
-	all x,y : User, z: Photo | x->z in posta 
+	all x,y : User, z: Photo | x->z in posts and y->z in posts implies x=y
+	
+	// or (equivalent to 2nd formula)
+	all x,y : User, z: Photo | x in posts z and y in posts z implies x=y
+	
+	// or (equivalent to 1st and 2nd formula)
+	all y: Photo | some posts y
+	all y: Photo | lone posts y
+	
+	// or (equivalent to 1st and 2nd formula)
+	all y:Photo | one posts y
 }
  
 
 pred inv2 {
 	// Users cannot follow themselves.
-
+	all x: User | x->x not in follows
 }
 
 
 pred inv3 {
 	// Users can see ads posted by everyone, 
 	// but only see non ads posted by followed users.
-
+	
 }