function zebra() {
	var table = document.getElementById("shimashima");
	var trs = table.getElementsByTagName("tr");
	for (var j=0; j < trs.length; j++) {
		if (j % 2) {
			addCSSClass(trs[j], 'alternate');
		}
	}
}

function addCSSClass (elem, className) {
	elem.className = (elem.className + " " + className);
}