code: def add_root(self, id_):
          
          if (bool(self._rls.get_relationships_by_genus_type_for_source(id_, self._relationship_type).available()) or
                  bool(self._rls.get_relationships_by_genus_type_for_destination(id_, self._relationship_type).available())):
              raise errors.AlreadyExists()
          self._assign_as_root(id_)
docs: Adds a root node.
  
          arg:    id (osid.id.Id): the ``Id`` of the node
          raise:  AlreadyExists - ``id`` is already in hierarchy
          raise:  NotFound - ``id`` not found
          raise:  NullArgument - ``id`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          *compliance: mandatory -- This method must be implemented.*