=head2 insert_node( $rs, $id?, \%data? )

Insert a new first node in $rs.

In case $id is given, the new node will be inserted as last node in the node
identified by $id (now refered as node($id).

Additional %data of the new node can be given as hashref.

By default, if no $id is given, this inserts a root node. The ident of the root
node wil be 'root'.

=cut