切换tab，并且动态添加标签

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  document.getElementsByClassName = function(className, element) {
   var children = (element || document).getElementsByTagName('*');
   var elements = new Array();
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    var classNames = child.className.split(' ');
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   return elements;
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 }
 var newNode = document.createElement("i");
 var _titles = document.getElementsByClassName('title');
 var _items = document.getElementsByClassName('_items');
 var _as = document.getElementsByClassName('title_a');
 _items[0].style.display = 'block';
 _titles[0].insertBefore(newNode, _as[0]);
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   _titles[i].index = i; 
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     _items[j].style.display = 'none';    
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   _items[this.index].style.display = 'block';
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