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Re-index sorting with Core Data, NSTreeController and NSOutlineView
- (void)reIndex {
    NSUInteger count = [[_arrayController arrangedObjects] count];
    NSArray *array = [NSArray arrayWithArray:[_treeController arrangedObjects]];

    for (int i = 0; i < count; i++) {
        id item = [array objectAtIndex:i];
        [item setValue:[NSNumber numberWithInt:i] forKey:@"sortIndex"];
    }
}