//c  seems to be what you are looking for.


However

> The compact XPath expression could then be actually compiled into the
> complete xpath expression at compilation time
you can't expand //c to /a/b/c at compile time as teh path osn't known
> until run time, as it varies on the document supplied at run time.

In XSLT2, with a schema aware processor, in some limited situations, you
may be able to use knowledge of a schema to make such optimsations
but in general this is not possible.

David