Index: src/OFMutableSet.h
==================================================================
--- src/OFMutableSet.h
+++ src/OFMutableSet.h
@@ -18,10 +18,14 @@
 
 /**
  * \brief An mutable unordered set of unique objects.
  */
 @interface OFMutableSet: OFSet
+{
+	unsigned long mutations;
+}
+
 /**
  * \brief Adds the specified object to the set.
  *
  * \param object The object to add to the set
  */

Index: src/OFMutableSet.m
==================================================================
--- src/OFMutableSet.m
+++ src/OFMutableSet.m
@@ -19,19 +19,63 @@
 #define OF_MUTABLE_SET_M
 
 #import "OFMutableSet.h"
 #import "OFDictionary.h"
 #import "OFNull.h"
+#import "OFAutoreleasePool.h"
 
 @implementation OFMutableSet
 - (void)addObject: (id)object
 {
 	[dictionary _setObject: [OFNull null]
 			forKey: object
 		       copyKey: NO];
+
+	mutations++;
 }
 
 - (void)removeObject: (id)object
 {
 	[dictionary removeObjectForKey: object];
+
+	mutations++;
+}
+
+- (int)countByEnumeratingWithState: (of_fast_enumeration_state_t*)state
+			   objects: (id*)objects
+			     count: (int)count
+{
+	OFAutoreleasePool *pool = state->extra.pointers[0];
+	OFEnumerator *enumerator = state->extra.pointers[1];
+	int i;
+
+	state->itemsPtr = objects;
+	state->mutationsPtr = &mutations;
+
+	if (state->state == -1)
+		return 0;
+
+	if (state->state == 0) {
+		pool = [[OFAutoreleasePool alloc] init];
+		enumerator = [dictionary keyEnumerator];
+
+		state->extra.pointers[0] = pool;
+		state->extra.pointers[1] = enumerator;
+
+		state->state = 1;
+	}
+
+	for (i = 0; i < count; i++) {
+		id object = [enumerator nextObject];
+
+		if (object == nil) {
+			[pool release];
+			state->state = -1;
+			return i;
+		}
+
+		objects[i] = object;
+	}
+
+	return count;
 }
 @end

Index: tests/OFSet.m
==================================================================
--- tests/OFSet.m
+++ tests/OFSet.m
@@ -15,12 +15,14 @@
  */
 
 #include "config.h"
 
 #import "OFSet.h"
-#import "OFAutoreleasePool.h"
 #import "OFArray.h"
+#import "OFAutoreleasePool.h"
+
+#import "OFEnumerationMutationException.h"
 
 #import "TestsAppDelegate.h"
 
 static OFString *module = @"OFSet";
 
@@ -104,10 +106,21 @@
 
 	if (i != 4)
 		ok = NO;
 
 	TEST(@"Fast enumeration", ok)
+
+	ok = NO;
+	@try {
+		for (OFString *s in mutableSet)
+			[mutableSet removeObject: s];
+	} @catch (OFEnumerationMutationException *e) {
+		ok = YES;
+		[e dealloc];
+	}
+
+	TEST(@"Detection of mutation during Fast Enumeration", ok);
 #endif
 
 	[pool drain];
 }
 @end