using an attribute's value as input for value-of?

I would like to use the value of an attribute which contains an
Expression as the input for an value-of.  Like this:


input xml
...
<elt attr=".//elta|.//eltb"/>
...


xsl stylesheet
...
<xsl:value-of select="@attr"/>
...

In this case, I know, the actual output will be ".//elta|.//eltb". 
How must I proceed to get the expression evaluated, and the resulting
value be output instead?



cheers,
-- 
Jakob.