var cur_lyr;	// holds id of currently visible layer

function swap(id) {
  showLayer(id);   
  cur_lyr = id;
 }

function showLayer(id) {
  if (document.getElementById(id).className == "hidden") {
  document.getElementById(id).className = "view";
  if (cur_lyr && cur_lyr!=id) {
  document.getElementById(cur_lyr).className = "hidden";}
  } else 
  {
  document.getElementById(id).className = "hidden";
  }
}