// From Michael Kay: http://markmail.org/message/vkb2vaq2miylgndu
//
// Underneath the s9api XPathExecutable is a net.sf.saxon.sxpath.XPathExpression.
XPathExpression xexpr = xexec.getUnderlyingExpression();
int pos = 0;
while (source.moreDocuments()) {
XdmNode doc = source.read();
pos++;
Item item = null;
try {
// Call createDynamicContext() on this to get an XPathDynamicContext object;
XPathDynamicContext xdc = xexpr.createDynamicContext(doc.getUnderlyingNode());
// call getXPathContextObject() on that to get the underlying XPathContext.
XPathContext xc = xdc.getXPathContextObject();
// Then call XPathContext.setCurrentIterator()
// to supply a SequenceIterator whose current() and position() methods return
// the context item and position respectively. If there's any risk that the
// expression will call the last() method, then it's simplest to make your
// iterator's getProperties() return LAST_POSITION_FINDER, and implement the
// LastPositionFinder interface, in which case last() will be implemented by
// calling the iterator's getLastPosition() method. (Otherwise last() is
// implemented by calling getAnother() to clone the iterator and calling next()
// on the clone until the end of the sequence is reached).
xsi.setPosition(pos);
xsi.setItem(doc.getUnderlyingNode());
xc.setCurrentIterator(xsi);
// Then evaluate the expression by calling iterate() on the
// net.sf.saxon.sxpath.XPathExpression object.
SequenceIterator<?> results = xexpr.iterate(xdc);
// FIXME: What if the expression returns a sequence?
item = results.next();
} catch (XPathException xe) {
throw new XProcException(xe);
}