* Collapsing flat tree returned from query to recursive structure.
* Node classes may use polymorphic inheritance.
-Moving of nodes is not yet implemented.
+ * Nodes and whole trees/subtrees can be moved around or removed
Known-to-work supported DBMS include `sqlite`_ (tested with 3.6.14),
`MySQL`_ (tested using both MyISAM and InnoDB with server version 5.1.34)