How to match namespaces instead of namespace prefixes ?

As we know different XML documents could have different namespace prefixes 
for the same namespace. E.g.:

doc1.xml:
<topelem xmlns:aaa="http://www.foo.com"; xmlns:bbb="http://www.bar.com"; ... >
...
<aaa:subelem bbb:myattr="1">
</topelem>

doc2.xml:
<topelem xmlns:xxx="http://www.foo.com"; xmlns:zzz="http://www.bar.com"; ... >
...
<xxx:subelem zzz:myattr="1">
</topelem>


Now I want to write a XSLT stylesheet which matches a subelem if the prepended
namespace prefix belongs to the namespace "http://www.foo.com";
regardless which current namespace prefix is used.

How do I implement this ?

In other words I am searching for a function similar to

<xsl:copy-of select="searchprefixfor(http://www.foobar.com):subelem" />

Is there such a function?
Is there a workaround?

Thank you
Matt

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