function show_layer (layerID) {
  obj = document.getElementById (layerID);
  if (!obj || !obj.style) return;
  obj.style.visibility = 'visible';
}

function hide_layer (layerID) {
  obj = document.getElementById (layerID);
  if (!obj || !obj.style) return;
  obj.style.visibility = 'hidden';
}