function add_selected_features(from, to) {
	
	var add;
	
	for(i = 0; i < from.length; i++) {
		if(from.options[i].selected) {
			add = true;
			for(j = 0; j < to.length; j++) {
				if(to.options[j].value == from.options[i].value) {
					add = false;
					break;
				}
			}
			if(add) {
				to.options[to.length] = new Option(from.options[i].text, from.options[i].value, false, false);
			}
		}
	}
	
}

function remove_selected_features(ele) {
	
	for(i = ele.length - 1; i > -1; i--) {
		if(ele.options[i].selected) {
			ele.options[i] = null;
		}
	}
	
}

function select_all_features(ele) {

	for(i = 0; i < ele.length; i++) {
		ele.options[i].selected = true;
	}
	
}