@@ -32,10 +32,11 @@
 @end
 
 @interface SimpleMutableSet: OFMutableSet
 {
 	OFMutableSet *_set;
+	unsigned long _mutations;
 }
 @end
 
 @implementation SimpleSet
 - (instancetype)init
@@ -115,19 +116,10 @@
 
 - (OFEnumerator *)objectEnumerator
 {
 	return [_set objectEnumerator];
 }
-
-- (int)countByEnumeratingWithState: (of_fast_enumeration_state_t *)state
-			   objects: (id *)objects
-			     count: (int)count
-{
-	return [_set countByEnumeratingWithState: state
-					 objects: objects
-					   count: count];
-}
 @end
 
 @implementation SimpleMutableSet
 + (void)initialize
 {
@@ -135,16 +127,39 @@
 		[self inheritMethodsFromClass: [SimpleSet class]];
 }
 
 - (void)addObject: (id)object
 {
+	bool existed = [self containsObject: object];
+
 	[_set addObject: object];
+
+	if (existed)
+		_mutations++;
 }
 
 - (void)removeObject: (id)object
 {
+	bool existed = [self containsObject: object];
+
 	[_set removeObject: object];
+
+	if (existed)
+		_mutations++;
+}
+
+- (int)countByEnumeratingWithState: (of_fast_enumeration_state_t *)state
+			   objects: (id *)objects
+			     count: (int)count
+{
+	int ret = [_set countByEnumeratingWithState: state
+					    objects: objects
+					      count: count];
+
+	state->mutationsPtr = &_mutations;
+
+	return ret;
 }
 @end
 
 @implementation TestsAppDelegate (OFSetTests)
 - (void)setTestsWithClass: (Class)setClass