Organized STree; added navigation methods (prev_kid, next_kid, ancestors, etc.)
BUGFIX: Didn't select root node when using the ancestor operator (' ')

Bug introduced with the :is-root() feature + bugfix
select() now return STreeCollection, which allows chaining select()s

This is a good way to increase specificity, or to force forward-search
Version 0.4.6
Added leaves() method, which returns a STreeCollection filtered by the head
Optimization: Match modifiers before elements

In my tests, run time was sped up by 5%