gistfile1.js

var sourceIndex = (function(){
  var root = document.documentElement, 
      indexOf = Array.prototype.indexOf;
  if ('sourceIndex' in root) {
    if (root.sourceIndex === 0) { // Opera
      return function(e) {
        return e.sourceIndex + 1;
      }
    }
    else if (root.sourceIndex === 1) { // IE
      return function(e) {
        return e.sourceIndex;
      }
    }
  }
  else if (indexOf && indexOf.call(document.getElementsByTagName('*'), document.documentElement) === 0) {
    return function(e) {
      return indexOf.call(e.ownerDocument.getElementsByTagName('*'), e) + 1;
    }
  }
})();