Suggest a combo-box design that allows for hierarchical selections.

Suppose: A user has a combo box filter and an associated combo box.

The combo-box selections are

* Dog

* Cat

* Fox

* Lizard

* Snake

* Gecko

The designer would like to allow the user to make netted selections. For example:

* Mammal

** Dog

** Cat

** Fox

* Reptile

** Lizard

** Snake

** Gecko

If the user selects the check box next to "Mammal" then all of the components are selected.