function changeClass(x,cls){
  x.className = cls;
}

function preview(x,tag){
  var el = x.parentNode.getElementsByTagName(tag).item(0);
  if( x.value == "+"){
    x.value = "-";
    el.style.display = "block";
  } else {
    x.value = "+";
    el.style.display = "none";
  }
}