How can I access specific subsets of a large NSDictionary in Cocoa?

@interface NSDictionary (dictionaryForClass)

  -(NSMutableDictionary *) dictionaryWithObjectsKindOfClass:(Class)myClass;

@end

@implementation NSDictionary (dictionaryForClass)

-(NSMutableDictionary *) dictionaryWithObjectsKindOfClass:(Class)myClass;
{
  NSMutableDictionary *ret = [[[NSMutableDictionary alloc] init] autorelease];

  for (id object in self) {
    if ([object isKindOfClass:myClass]) {
       [ret addObject:object];
    }
  }
  return ret;

}

@end