function check_fields(container) {

   fields = $(container).getElementsByTagName('input');
   for (i = 0; i < fields.length; i++) {
       fields[i].checked = true;
   }

}
Event.observe(window, 'load', function(e) {

	if($('cat-clicker')) {
		Event.observe('cat-clicker', 'click', function(e) {
			Event.stop(e);
			check_fields('categories');
		});
	}

	if($('pri-clicker')) {
		Event.observe('pri-clicker', 'click', function(e) {
			Event.stop(e);
			check_fields('priorities');
		});	
	}

});