oldLayer = "";
function showHide(layerName) {
	document.getElementById(layerName).style.display = "block";
	if (oldLayer != "") {
		document.getElementById(oldLayer).style.display = "none";
	}
	if (oldLayer == layerName) {
		oldLayer = "";
	} 
	else {
		oldLayer = layerName;
	}
}