>> It can be done. 
>
> what can be done?

The OP wrote: "  I'd like to conditionally add an attribute set to an 
element." The stylesheet I wrote does that. Of course, it always adds one, 
but it's conditional. I figured the OP could get to doing it with xsl:if 
on his own.

>
>> Consider the following stylesheet
>
> But that stylesheet doesn't use attribute sets at all.
> It writes out a result that happens to have an atribute called
> xsl:use-attribute-sets but the attribute sets defined in the stylesheet
> e are not used, are they?

I thought the point was to get the attribute set into the output element, 
which it did.

>
>>  (which you can test by  applying it to itself):
>
> but you get the same result if you remove
>
>   <xsl:attribute-set name="Oscar">
>     <xsl:attribute name="slob">yes</xsl:attribute>
>   </xsl:attribute-set>
>
>   <xsl:attribute-set name="Felix">
>     <xsl:attribute name="slob">no</xsl:attribute>
>   </xsl:attribute-set>
>
> from the stylesheet.

Yes. I put them in so that what I was referring to would be defined, not 
to use them. I thought the OP wanted that rather than wanting them to be 
expanded. I assumed he was going to do a further transformation later in 
his process.

Sorry about any confusion.

Jay Bryant
Bryant Communication Services
(presently consulting at Synergistic Solution Technologies)