DIH http input xpath syntax

[Lance Norskog]

The wiki page for the DIH handler mentions that the XML is parsed with a
streaming parser and that the xpath parser only handles a subset of the
xpath syntax. Which streaming parser is it and where would I find this
subset documented?  I tried a few things like the "the first entry" and
"length of data > 3" and nothing worked.  "/a/b/c" was all that worked for
me.
 
Tx,
 
Lance