function shm(id,tf) {
  var obj = document.getElementById(id).style;
  if(!tf) {
    if(obj.visibility=='hidden') {
      obj.visibility='visible';
      obj.position='relative';
    } else {
      obj.visibility='hidden';
      obj.position='absolute';
    }
  } else {
    obj.visibility='visible';
    obj.position='relative';
  }
}