function lightup(cell,active,on,bg) {
  
  off = "transparent";

  if(active) {
   cell.style.cursor='hand';
   cell.style.backgroundColor=on;
   if(bg)
    cell.style.background = 'url('+bg+')';

  }
  else {
   cell.style.backgroundColor=off;
   cell.style.background='transparent';
  }
}