key with restricting the scope of use attribute

I have a key that works when the match is anchored to
fixed path /a/b/c

  <xsl:key name="MyKey" match="/a/b/c" use="?"/>

My ultimate goal is to recursively process //a/b/c
using a single key using a breadth first or depth first 
algorithm. If the node I am currently processing is held 
in a variable $CurrentNode, how would you construct 
the key to restrict its matching to just this node? In other
words, I need to form the use attribute knowing $CurrentNode
so that match="//a/b/c" does not match anthing other than
$CurrentNode.

The source XML is too long and complicated to post, but I 
could come up with an example if the above description is 
not adequate. Again, I am looking for a general technique 
with speed as the primary consideration.

Regards,

Dan

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