function expand_category(category_id) {
	var li_elements = document.getElementsByTagName("li");
	var num_elements = li_elements.length;

	for (var i = 0; i < num_elements; i++) {
		// expand selected category
		if (li_elements[i].className == "category_id_" + category_id)
			li_elements[i].style.display = "block";

		// collapse all other categories
		else if (li_elements[i].className.indexOf("category_id") != -1)
			li_elements[i].style.display = "none";
	}
}