code: public int addSection (String label, ListModel model)
      {
          add(new JLabel(label));
          add(new JList(model));
          return getSectionCount()-1;
      }
docs: Adds a section to this collapsible list.
  
  @param label the title of the section.
  @param model the list model to use for the new section.
  
  @return the index of the newly added section.