<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_root_books(self):
 """Gets the root books in the book hierarchy. A node with no parents is an orphan. While all book ``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: (osid.commenting.BookList) - the root books raise: OperationFailed - unable to complete request raise: PermissionDenied - authorization failure *compliance: mandatory -- This method is must be implemented.* """

        # Implemented from template for
        # osid.resource.BinHierarchySession.get_root_bins
        if self._catalog_session is not None:
            return self._catalog_session.get_root_catalogs()
        return BookLookupSession(
            self._proxy,
            self._runtime).get_books_by_ids(list(self.get_root_book_ids()))