XPath Context & Scope for string-replace

Looking at http://www.w3.org/TR/xproc/#c.string-replace

I wonder what the XPath context is for the evaluation of the XPath
expression?  It
probably should be the matched node.

Also, what is the allowable scope of the expression?  The whole document?

So, if I write "concat(../foo,.)" I've blow streaming for that step.
That's fine and that
flows along with our "if you can figure out streaming, good for you" model.


-- 
--Alex Milowski
"The excellence of grammar as a guide is proportional to the paucity of the
inflexions, i.e. to the degree of analysis effected by the language
considered."

Bertrand Russell in a footnote of Principles of Mathematics