function clearUncheckedQuantities(form) {
    var qtys = form.elements["quantity"];
    var len = qtys.length;
    for (var i = 0; i < len; i++) {
	var id = qtys[i].id;
	var checkboxElem = document.getElementById("chk" + id.substring(3));
	if (checkboxElem && !checkboxElem.checked)
	    qtys[i].value = "0";
    }
}