function applyCascadingDropdown(sourceId, targetId) {
	var source = document.getElementById(sourceId);
	var target = document.getElementById(targetId);
	if (source && target) {
		source.onchange = function() {
			displayOptionItemsByClass(target, source.value);
		}
		displayOptionItemsByClass(target, source.value);
	}
}

function displayOptionItemsByClass(selectElement, className) {
	if (!selectElement.backup) {
		selectElement.backup = selectElement.cloneNode(true);
	}
	var options = selectElement.getElementsByTagName("option");
	for(var i=0, length=options.length; i<length; i++) {
		selectElement.removeChild(options[0]);
	}
	var options = selectElement.backup.getElementsByTagName("option");
	for(var i=0, length=options.length; i<length; i++) {
		if (options[i].className==className)
			selectElement.appendChild(options[i].cloneNode(true));
	}
}

function applyCascadingDropdowns() {
	applyCascadingDropdown("topics", "categories");
}

window.onload=applyCascadingDropdowns;