> In line with the question whether to prefer XPath or XSLT 
> logic, can we see an example where we'd actually prefer to do 
> it in XPath if we could? (I'd welcome entries from anyone.)

One case is a sort based on a lookup field:

<xsl:sort select="$lookup[data=current()/@x]"/>

current() here is a bit of a hack: it's in effect a built-in variable. It
allows you to do a 2-way join without resorting to explicit variables, but
fails when you want a 3-way join:

<xsl:sort select="$lookup1[let $x:=. return data=$lookup2[data=$x/@x]]"/>

In practice I've found that the variable you want to bind here is almost
invariably bound to the context item, which means you can write it as

<xsl:sort select="$lookup1[for $x in . return data=$lookup2[data=$x/@x]]"/>

but it seems an ugly hack to use "for" to iterate over a singleton, just
because you want to bind a variable.

At one time I was trying to come up with some explicit syntax for binding
the context item to a variable, for example

<xsl:sort select="$lookup1[data = with $x do ($lookup2[data=$x/@x])]"/>

but it's not significantly better than using "let $x:=.".

Michael Kay
http://www.saxonica.com/