fixed scroll

	var $comment = $('.def_index'),
	top = $comment.offset().top - parseFloat($comment.css('marginTop').replace(/auto/, 0)),
	$window = $(window);
 
	$window.scroll(function (event) {
	// what the y position of the scroll is
	var y = $window.scrollTop();
 
	// whether that's below the form
	if (y >= top) {
	// if so, ad the fixed class
	$comment.addClass('fixed');
	} else {
	// otherwise remove it
	$comment.removeClass('fixed');
	}
	});