Put a wrapper around a series of elements

This seems like it should be easy but I'm stumped.  I
tried FAQ, books. . .

Have

<a>
 <b>some text</b>
 <b>some more text</b>
 <b>some other text</b>
 <b>some last text</b>
<c>some c text</c>
</a>

Want

<a>
 <bset>
  <b>some text</b>
  <b>some more text</b>
  <b>some other text</b>
  <b>some last text</b>
 </bset>
<c>some c text</c>
</a>

I successfully tested for the first and last <b>, but
couldn't output unmatched elements. . .

Is there a better way?


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