Mutable copying

    NSArray *items = @[@1, @2, @"Three"];
    NSDictionary *d = @{@"items" : items};
    NSMutableDictionary *md = [d mutableCopy];
    id mitems = md [@"items"];

    NSLog (@"items %p == %p mutable items? %i", items, mitems, items == mitems);

    // fail: -[__NSArrayI addObject:]: unrecognized selector
    //[mitems addObject: @"Four"];

    NSMutableDictionary *md2 =
      (id)CFBridgingRelease
        (CFPropertyListCreateDeepCopy (NULL, (CFPropertyListRef)d,
                                       kCFPropertyListMutableContainers));

    id mitems2 = md2 [@"items"];
    NSLog (@"items %p == %p mutable items? %i", items, mitems2, items == mitems2);

    // OK
    [mitems2 addObject: @"Four"];