how to get the position of first special element?

ALL,

For example, 

<element>
<A/>
<B/>
<A/>
<C/>
<C/>
<D/>
</element>

I want to get the position first element whoes name is not A and B, but C & 
D. So it's 4 for this tree segment.
How can I write the xslt to figure out this ?

Thanks,
Chen Yi

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