var elem = "TR";

 window.onload = function(){
  if(document.getElementsByTagName){
		var el = document.getElementsByTagName(elem);
		for(var i=0; i<el.length; i++){
		  if(el[i].parentNode.parentNode.className.indexOf("tbl") != -1){
				 if(i%2 == 1){
					  el[i].className = "on";
					  el[i].onmouseout  = function(){
						 this.className = "on";
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				} else {
					  el[i].className = "off";
					  el[i].onmouseout  = function(){
						 this.className = "off";
					  }
				}
			  el[i].onmouseover = function(){
				 this.className = "hover";
			  }
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	  }
	}
}