function popup(url,param,width,height)
      {
        var win = null;
        var venster;
        venster = Math.round(Math.random()*10000);
        if( param.length == 0 )
        {
          param = "?breedte="+width+"&hoogte="+height+"&elem="+venster;
        }
        else
        {
          param = param+"&breedte="+width+"&hoogte="+height+"&elem="+venster;;
        }
        LeftPosition = (screen.width) ? (screen.width-width)/2 : 0;
        TopPosition = (screen.height) ? ((screen.height-height)/2)-20 : 0;
        settings = "height="+height+",width="+width+",top="+TopPosition+
                   ",left="+LeftPosition+",scrollbars=yes,resizable,location=no";
        win = window.open(url+param,venster,settings);
      }