last_gm = 'lyr0';
function show(layerName) {
document.getElementById(layerName).style.display = '';
}

function hide(layerName) {
document.getElementById(layerName).style.display = 'none';
}
function show_next(next_gm) {
document.getElementById(last_gm).className = 'off';
var curr = document.getElementById(next_gm);
curr.className='on';
hide(last_gm+'_preview');
show(next_gm+'_preview');
last_gm=next_gm;
}