increment value

Hello,

I want to increment a value to concat to a string. 
For example:

        <a>
              <b>1</b>
        <b>2</b>
      </a>

      <a>
              <b>3</b>
        <b>4</b>
      </a>
      
The output would be:
      
        <b id="b1"/> 
        <b id="b2"/> 
        <b id="b3"/> 
        <b id="b4"/> 
        .
      
Can anybody help,
      
Thanks
      
Ricardo Saraiva.



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