Return the value of the ``./c:grouping{val=?}`` attribute, taking
        defaults into account when items are not present.

def grouping_val(self):
        """
        Return the value of the ``./c:grouping{val=?}`` attribute, taking
        defaults into account when items are not present.
        """
        grouping = self.grouping
        if grouping is None:
            return ST_Grouping.CLUSTERED
        val = grouping.val
        if val is None:
            return ST_Grouping.CLUSTERED
        return val