[development] fill the multiple select list box

Dear all, i have created a mulitple listbox using '#type' => 'select'. i have some buttons 'add', 'delete', 'clear'. when i click 'add' button i want to add some text in listbox as many as i want. if i click 'delete' i want to remove the items one by one. And if i click 'clear' the listbox should be empty. can you please help me to solve this problem. Thank you Jayakumar