#22 Let extended translators override what XPathExpr class is used
#19 Use the built-in lang() XPath function for implementing the :lang() pseudo-class with XML documents. This is probably faster than ancestor-or-self::.
Bug fixes:
#14 Fix non-ASCII pseudo-classes. (Invalid selector instead of crash.)
#20 As per the spec, elements containing only whitespace are not considered empty for the :empty pseudo-class.