[xsl] XPath expression to check that there are no intervening elements?

Hi Folks,

This XML has a solid block of <B> elements:

<Document>
    <A/>
    <B/>
    <B/>
</Document>

This XML has an intervening <C> element:

<Document>
    <A/>
    <B/>
    <C/>
    <B/>
</Document>

I need an XPath expression to return a Boolean value:

        Return true if there is one solid block of <B> elements
                (no intervening elements).
        Otherwise, return false.

I created a horrendous XPath expression to solve it:

count(B) eq (B[last()]/count(preceding-sibling::*)+1 - 
B[1]/count(preceding-sibling::*))

Can you provide a better (simpler, more efficient) XPath expression?

/Roger
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