XSL: use of id(...) ?

Hi,

I am trying to use id(...), but in the best case, nothing happens.

      <xsl:apply-templates select="id('bla')">

How is it possible to use any computed value instead of bla, e.g.
for XML subelements of the form

<ref_to_city ref="foo1"/>
<ref_to_city ref="foo2"/>
<ref_to_city ref="foo3"/>

  <xsl:apply-templates select="id(current()/ref_to_city/@ref)">

to search for applicable templates to the cities which are 
referenced?

Can variables be used in  "id($var)" ?

Or -- if the whole way I try to use id(...) is wrong -- where 
can it be used?

Wolfgang


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