$(document).ready(function(){
        $('li:has(ul)')
          .click(function(event){
            if (this == event.target) {
              $(this).css('list-style-image',
                (!$(this).children().is(':hidden')) ?
                  'url(./images/plus.gif)' : 'url(./images/minus.gif)');
              $(this).children().toggle('fast');
            }
            return true;
          })
          .css({cursor:'pointer',
                'list-style-image':'url(./images/plus.gif)'})
          .children().hide();
        $('li:not(:has(ul))').css({
          cursor: 'default',
          'list-style-image':'none'
        });
      });