- Support DAGs.
 - Tree::Visitor
 - Come up with a better name for attach_before and
   attach_after.
 - Support multiple columns for ordering.
 - Declare both the parent column and the position column
   in one call.
 - Add an ancestors() and descendants() method with
   support for resultset cacheing.
 - Not all methods are covered by the tests.
 - has_ancestor() method.
 - DBIx::Class::Tree::NestedSet