@abc.abstractmethod<EOL><INDENT>def get_existing_spatial_unit_values(self):<DEDENT>
return<EOL>
Gets the existing spatial unit values.

        If ``has_value()`` and ``is_required()`` are ``false,`` then
        these values are the default values ````. If ``is_array()`` is
        false, then this method returns at most a single value.

        :return: the existing spatial unit values
        :rtype: ``osid.mapping.SpatialUnit``
        :raise: ``IllegalState`` -- syntax is not a SPATIALUNIT or ``is_value_known()`` is ``false``

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