6.1 addEvent

function addEvent(elm, evType, fn, useCapture) {

  if (elm.addEventListener) {

    elm.addEventListener(evType, fn, useCapture);//DOM2.0

    return true;

  }

  else if (elm.attachEvent) {

    var r = elm.attachEvent('on' + evType, fn);//IE5+

    return r;

  }

  else {

    elm['on' + evType] = fn;//DOM 0

  }

}