function tc_addTipListener(){
  if(document.addEventListener){
    document.addEventListener("TipEvent", 
      function(evt){
        tipcup_ontip(evt.target.getAttribute("tipAmount"));
      }, false, true);
  }
}

function isTipCupUser(){
  var element = document.createElement("TipCupTester");
  element.setAttribute("hasTipCup", "false");
  document.documentElement.appendChild(element);
  var evt = document.createEvent("Events");
  evt.initEvent("TipCupEvent", true, false);
  element.dispatchEvent(evt);
  return (element.getAttribute("hasTipCup") != "false");
}