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

Saxon-SA will rewrite //c as /a/b/c provided that the type of the context
node is statically known. It is generally good practice to declare type
information when writing schema-aware stylesheets, for example

<xsl:template match="document-node(schema-element(fpml:FPML))">
  <xsl:value-of select="//x"/>
</xsl:template>

The type information can be used both for optimization and for better error
checking: with a complex schema like FPML, it's very useful to be able to
abbreviate your path expressions (which can otherwise be immensely long)
without incurring any performance penalty.

Michael Kay
http://www.saxonica.com/