var layerflg={
  'parent':false,
  'child' :false
};
function showLayer(id,ref){
  document.getElementById(id).style.visibility = 'visible';
  layerflg[ref]=true;
}
function hideLayer(id,ref){
  layerflg[ref]=false;
  if(ref=='parent' && layerflg.child || ref=='child' && layerflg.parent) return;
  document.getElementById(id).style.visibility = 'hidden';
}