optional children

Hi all,

I want to output <foo/> if and only if element <a> has child <b>, with
optional children <c> and <d>.

So for example

<a>
  <b/>
</a>

should output <foo/>

<a>
  <b/>
  <c/>
  <d/>
</a>

(or any combination of)

should output <foo/>

<a>
  <b/>
  <e/>
</a> 

should not.

I think this is more difficult than it looks, the optional children
really mess things up... but I may be wrong :)  

cheers
andrew

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