function explain(url) {
  if(url != null) {
    explainWin=open(url, "notation",
                    "width=500,height=300,status=yes,toolbar=no,menubar=no,scrollbars=yes,alwaysRaised=yes");
  }
  else
    {
      alert("Target URL" +  url + "does not exist");
    }
}