function flipLayer (layername) 
{
// turn all the layers off one by one
document.getElementById('layer1').style.display = 'none';
document.getElementById('layer1a').style.display = 'none';
document.getElementById('layer1b').style.display = 'none';
document.getElementById('layer2').style.display = 'none';
document.getElementById('layer2a').style.display = 'none';
document.getElementById('layer2b').style.display = 'none';


//turn the layer you want to show on
document.getElementById(layername).style.display = 'block';
}