def get_root_repositories(self):
return<EOL>
Gets the root repositories in the repository hierarchy.

        A node with no parents is an orphan. While all repository
        ``Ids`` are known to the hierarchy, an orphan does not appear in
        the hierarchy unless explicitly added as a root node or child of
        another node.

        :return: the root repositories
        :rtype: ``osid.repository.RepositoryList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method is must be implemented.*