[xsl] Need your recommendation please for an XPath expression

["Costello, Roger L. costello@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Folks,

I need an XPath expression which returns a Boolean value:

	If there is an A then return true only if there is a B.
     	If there is no A then return true.

For example, the XPath expression should return true for this:

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

The XPath should return false for this:

<Document>
    <A/>
</Document>

Which XPath expression is best (most efficient)?

(a)   if (A) then exists(B) else true()

(b)  (A and B) or (not(A))

(c)  something else (what?)

/Roger