Idea collection

* XPath localization paths
  - [ ] ancestor
        - http://doc.ddart.net/xmlsdk/htm/xpath_syntax2_3prn.htm
        - examples:
          * Find the nearest book ancestor of the current element:
            ancestor::book[1]
          * Find the nearest ancestor book that has an author element:
            ancestor::book[author][1]
          * Find the nearest ancestor author element that is contained
            in a book element:
            ancestor::author[parent::book][1]        
  - [ ] preceding-sibling
  - [ ] preceding
        - might not work well with hashes
        - still useful, as data could be tied to Tie::IxHash?
        - else ignore the wrong order, it's the user's responsibility
          to (not) use it this way, as it is the same with ordinary
          perl % and @
* Functions for node collections
  - [ ] position()
  - [ ] last()
  - [ ] count()
  - [ ] id()
  - [ ]