You can introduce intermediate classes: Animal > GracefulAnimal > Cat. The problem is if there's other branches that are also graceful that there's no way to in

You can introduce intermediate classes: Animal > GracefulAnimal > Cat. The problem is if there's other branches that are also graceful that there's no way to include just the right subset. That's where composition is better in that a Graceful aspect can be mixed-in. It's basically the same problem of categorizing with labels in a single tree structure. You often find you want to group lower subtrees without including their parent nodes. That's why Gmail labels are better than normal email folders.