XPath context for idValue evaluation

Is the XPath context for the evaluation of the idValue attribute of
idValueRule the same as for relative selectors? I suspect that it is, but
it is not in the spec. Since relative selectors have size and position set,
this would provide a mechanism for creating unique IDs, as well as an easy
unit test for correct implementation of selector XPath contexts, using
something like this:

<its:idValueRule selector="//par" idValue="concat('par_', position(),
'_of_', last())"/>

Nathan