function unhide(id){
  var divs = document.getElementsByTagName("div");
  for(var i=0; i<divs.length; i++){
     if(divs[i].style.display == 'block'){
        //Some DIV is visible. You can stop and exit function or hide it
        divs[i].style.display = 'none';
     }
  }
  document.getElementById(id).style.display = 'block';
}