@abc.abstractmethod<EOL><INDENT>def get_hierarchies(self):<DEDENT>
return<EOL>
Gets all hierarchies.

        In plenary mode, the returned list contains all known
        hierarchies or an error results. Otherwise, the returned list
        may contain only those hierarchies that are accessible through
        this session.

        :return: a list of ``Hierarchies``
        :rtype: ``osid.hierarchy.HierarchyList``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

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