@abc.abstractmethod<EOL><INDENT>def get_spatial_unit_set(self):<DEDENT>
return<EOL>
Gets the set of acceptable spatial unit values.

        :return: a set of spatial units or an empty array if not restricted
        :rtype: ``osid.mapping.SpatialUnit``
        :raise: ``IllegalState`` -- syntax is not a ``SPATIALUNIT``

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