//Whoohaa!
XPathExpression expr = nav.Compile("set:distinct(//author)");
expr.SetContext(new ExsltContext(doc.NameTable));
XPathNodeIterator authors = nav.Select(expr);
while (authors.MoveNext())
Console.WriteLine(authors.Current.Value);
EXSLT's set:distinct in XPath-only selection. Sweet.Coming soon, watch announcements!

Illustrates how EXSLt function scan be used in XPath-only environment.
Sorry to pose the stupid question. But what does that piece of code accomplish ?
Great news Oleg!!! I'm looking forward this enhancements to make them available in Schematron.NET too!